A child is somewhere between a third and a half of an adult, and the honest answer is that nobody can tell you which. Four published equivalence scales — built by statistical agencies to measure what a household needs, which is the question sitting underneath this one — answer it four different ways: 0.3 of an adult, 0.5 of an adult, half an adult passed through an economy-of-scale exponent, and one scale that refuses to count children separately at all. Run the same beach house through all four and the family’s share of the bill lands between 58% and 61%. The folk rule your group chat already invented sits almost exactly in the middle.
That near-tie holds when the households are the same shape, and it breaks when they are not: add one person travelling alone among couples and the choice of scale swings their own bill by about a third. So the useful finding is a conditional one. Whether kids count as half is rarely the argument worth having. Whether the households are the same size — and whether anyone said the rule out loud before the deposit — is.
Where does the “kids count as half” rule come from?
Nowhere in particular, which is the problem. It circulates as received wisdom. A 2026 group-travel guide aimed at US groups of four to fifteen people states it plainly — assign shares like “adults = 1, kids under 10 = 0.5, or a family of 4 = 3 shares” — crediting the figures to another travel blog rather than to any derivation. Nothing explains where the 0.5 came from, or why the cutoff is ten years old rather than eight or fourteen.
Source: How to Split Group Trip Costs Fairly: Workflows and Spreadsheet Templates, Spark, 2026.
That’s not a criticism of the advice so much as a description of a whole genre. The rule survives because it is directionally obvious — a six-year-old does eat less and use less house than an adult — and because 0.5 is the easiest fraction to compute in your head at a kitchen counter. It is a folk equivalence scale: a real economic object, invented independently by every group that has ever needed one, and never once checked against the literature that exists on precisely this.
Why can’t you just itemize what the kids used?
Because the costs that start the argument are the ones that cannot be cut into portions. This is the structural difference between a restaurant dinner and a shared house, and it’s why the weighting question only bites in one of them.
At a restaurant, the kids-versus-adults problem has a clean solution: itemize. When two couples share a table and one brought children who ordered off the kids’ menu, splitting evenly makes the childless couple subsidize meals they didn’t eat — and the fix is simply to put each dish on whoever ordered it. The receipt already contains the answer. Nobody needs a theory of childhood consumption; they need to read the lines.
A house has no lines. A $3,200 week in a rental is one indivisible good, consumed jointly, with no per-person meter. So are the rental car, the Wi-Fi, and the cleaning fee. Some of these have a better basis than headcount if you look for one — a house can be split by bed and night, groceries by category — and where such a basis exists, use it; household weighting is the fallback for what’s left.
Shared grocery runs are not a hypothetical stand-in for real costs, either — they show up constantly in what people actually scan. When the cost can’t be itemized, the only lever left is how much each person counts — and that is exactly the question an equivalence scale answers.
Grocery share: splitty’s own scanned receipts (US-leaning sample, 2026 snapshot), share of receipts by merchant category, rounded. This is splitty’s sample, not a national spending survey.
The dividing line: if the cost has line items, itemize it and skip this article. If it’s one number for one house, you need a weighting rule — and you’re better off borrowing a published one than inventing a fresh one at 11pm in a group chat.
What is an equivalence scale?
An equivalence scale converts a household of any size and composition into a number of “equivalent adults,” so that households of different shapes can be compared on the same footing. As the OECD describes it, each household type is assigned a value in proportion to its needs — which is how statistical agencies adjust household income before measuring poverty and income inequality. It is, structurally, the same question as: how much of this house does each family owe?
The logic rests on economies of scale in consumption. As the US Bureau of Labor Statistics puts it, a family of two is assumed to need more income than a single person, but not double — because they share. Two people share one kitchen, one router, one heating bill. In a rental house, they also share one living room and one view.
Sources: The OECD Approach to Measuring Income Distribution and Poverty, Förster & Mira d’Ercole, OECD; and Differences across Place and Time in Household Expenditure Patterns: Implications for the Estimation of Equivalence Scales, Daley, Garner, Phipps & Sierminska, U.S. Bureau of Labor Statistics Working Paper 520, 2019.
That single idea — sharing makes each additional person cheaper than the last — is what the folk rule misses. “Adults = 1” charges the second adult in a household exactly as much as the first, as though they arrived in separate houses. Every published scale disagrees.
The four scales, and what each says a child is worth
Four scales cover most of the practical ground — the OECD lists the first three among the most commonly used, and the fourth is the one the US government actually runs on. They were built by different institutions, for different countries, using different methods — and they give different answers.
The OECD-modified scale, adopted by Eurostat in the late 1990s, assigns 1.0 to the first adult, 0.5 to each additional person aged 14 or over, and 0.3 to each child under 14. The older “Oxford” scale — which the OECD had floated in 1982 for countries without one of their own — is more generous to extra bodies: 1.0, then 0.7 per additional adult and 0.5 per child. The square root scale, used in recent OECD cross-country comparisons, throws out age entirely and simply divides by the square root of household size, so a household of four is assumed to need twice what one person needs.
Sources: Glossary: Equivalised disposable income, Eurostat; and The OECD Approach to Measuring Income Distribution and Poverty, Förster & Mira d’Ercole, OECD.
The American entry is the most interesting for our purposes. The US Census Bureau’s Supplemental Poverty Measure uses a three-parameter scale, and its formula for a two-parent family is:
Source: Supplemental Poverty Measure: Technical Documentation, U.S. Census Bureau — which is also where the official measure’s own approach is described: it adjusts thresholds by family size, number of children and adults, and whether the householder is 65 or older, a different mechanism. The SPM is a supplement, not a replacement: it “does not replace the official poverty measure” (Fox & Burns, The Supplemental Poverty Measure: 2020, U.S. Census Bureau).
Look closely at that first term. Inside the parentheses, the US Census Bureau’s Supplemental Poverty Measure counts a child as exactly half an adult. The group-chat rule is not a folk corruption of the expert one — on that parameter, it is the expert one. What the folk rule drops is everything to the right: the exponent that then discounts the whole household for sharing. (Because the exponent is non-linear, 0.5 is an input, not the child’s finished weight — and a single parent’s first child enters at 0.8 instead.)
Running one beach house through all four
Take an illustrative case: a $3,200 week, split between two households. The Reyes family is two adults and two children under 10. The Chens are two adults, no kids. Every scale below is applied the same way — compute each household’s share value, then divide the house in proportion.
The share values swing wildly — the family is worth 3.00 units under the folk rule and 2.00 under the square root scale, a 50% difference. And yet:
Every method puts the family between 58.3% and 61.4% of the house. The folk rule’s 60.0% is not an outlier; it’s the median. On a $3,200 rental, choosing the most family-friendly published scale over the least moves about $97 — roughly 3% of the total, or one dinner out.
Illustrative scenario, not survey data. Share values computed from the scale definitions cited above; dollar figures rounded to the nearest dollar. The SPM row uses (2 + 0.5 × 2)0.7 = 2.16 for the family and the two-adult form (2)0.5 = 1.41 for the couple.
Why do such different scales give such similar answers?
Because equivalence scales are relative. Splitting a fixed cost doesn’t care about the absolute size of anyone’s share value — only about the ratio between them. A scale that is stingy toward extra people is stingy toward both households, and most of the effect cancels in the division.
The residual differences come from where each scale places children relative to adults. The Oxford scale is the most expensive for families (children at 0.5 against extra adults at 0.7) and the OECD-modified is the cheapest (0.3 against 0.5). That gap is real, but between two similarly-shaped households it is worth less than the cost of the argument. The cancellation is a property of this composition, though, not a law — it depends on both households having the same number of adults. Break that symmetry and the scales stop agreeing, as the next section shows.
Where the folk rule actually goes wrong
Not on the children. On the second adult.
Each of the four scales here discounts additional adults: 0.5 under OECD-modified, 0.7 under Oxford, and implicitly under both the square root and the SPM exponent. (A strict per-capita split, which the OECD lists as the no-sharing extreme, is the one rule that doesn’t.) The folk rule discounts none of them either. Its adults are 1.0 each, all the way up.
This cancels out in the beach house above, where both households happen to have two adults. It stops cancelling the moment household sizes diverge — and then it moves real money. Take the same $3,200 house shared by three couples and one person travelling alone. The solo traveler is worth 1.0 under every scale, because the first adult always is. Only the couples move.
Note which way this runs. Discounting the second adult shrinks every couple’s weight while the solo traveler’s stays at 1.0, so the published scales push the solo traveler’s share up, not down — from 14.3% under the folk rule to 19.1% under the square root scale. The folk rule is not harsh on the person who came alone. It is unusually generous to them, and correspondingly hard on everyone who arrived in a pair.
Same illustrative $3,200 house; share values computed from the scale definitions cited above, dollars rounded. The square root and Census SPM rows coincide here because both value a two-adult household at √2 ≈ 1.41.
There is some evidence the true discount is steeper still. Estimating equivalence scales for eight countries from household expenditure data, Daley, Garner, Phipps and Sierminska report considerable variation in economies of scale across countries and find them generally larger than the square root scale implies — though that is a finding about measured expenditure patterns, not about how much a specific group of friends actually shares in a rental house.
If you change one thing: keep “kids = 0.5,” it’s fine. Notice instead that when households are different sizes, the folk rule quietly bills the couples for sharing they are already doing — and hands the discount to whoever came alone.
What this borrows — and what it doesn’t
Two honest caveats, because this article is applying a tool outside the job it was built for.
These scales measure needs, not fair shares
Equivalence scales were built to compare living standards across households — to decide who is poor, and by how much. They were not built to allocate a jointly consumed good between guests. Borrowing them for a beach house is an analogy, and a defensible one, because both problems ask the same underlying question: how much does an extra person actually add? But no statistical agency has blessed this use, and this article isn't claiming one has.
The experts don't agree either
The OECD is unusually candid about this. Its own methodology paper notes that equivalence scales are 'to some extent, conventional, rather than based on the analysis of consumption expenditure,' and states plainly that there is no universally accepted method for determining them — and that the OECD recommends no equivalence scale for general use. The spread you saw in the table isn't sloppiness. It's the actual state of the field.
The underlying measurement problem is genuinely hard. In their 1986 study of how to measure what children cost, Angus Deaton and John Muellbauer examined the two classical methods — Engel’s food-share approach and Rothbarth’s adult-goods approach — and concluded that true costs are generally overstated by Engel’s method and understated by Rothbarth’s. Under the most extreme assumptions, they found, the Engel method makes children about four times as expensive as the Rothbarth method does — a spread they illustrate with Sri Lankan and Indonesian survey data, and one that measures disagreement between two estimation methods rather than uncertainty about any particular American holiday.
Source: On Measuring Child Costs: With Applications to Poor Countries, Deaton & Muellbauer, Journal of Political Economy, 1986.
That is a fourfold disagreement between two respectable methods, in the peer-reviewed literature, on the central quantity — and it is a disagreement about measuring what children cost their parents, not about what anyone owes for a beach house. The transferable lesson is narrow but real: if the specialists cannot pin this number down, a group chat is not going to either, and precision is the wrong thing to chase. Pick a rule, say it, move on.
How to actually split a trip with mixed households
Separate the divisible from the indivisible
Restaurant meals, activities, and anything with a receipt get itemized — put each line on whoever consumed it. Only the genuinely joint costs (the house, the car, the cleaning fee, the shared groceries) need a weighting rule at all. Most trips shrink to a much smaller weighting problem once you do this.
Pick one published scale — and only one — then say it out loud
Two workable choices, not a blend. Either the OECD-modified scale ('the first adult counts 1, every other adult 0.5, kids under 14 count 0.3'), or the square root scale, which needs no table at all: a household counts as the square root of its size, so a family of four is 2, a couple is 1.41, and someone travelling alone is 1. Say which one before money moves. A rule named in advance is far easier to argue with than a number someone has already paid.
Whichever you pick, discount the second adult
This is the one real correction the literature makes to the folk rule, and it only bites when households are different sizes — it credits couples and families for the sharing they already do, which raises the relative share of anyone travelling alone. Both scales in step 2 do this automatically. The folk 'adults = 1, kids = 0.5' does not, which is the single thing worth changing about it.
Settle the weights before the deposit
The deposit is when the money actually moves and when one person's card absorbs the whole thing. Agreeing on shares afterward means renegotiating a number someone has already paid — which is the conversation this entire article exists to prevent.
The neighboring problems have their own answers worth borrowing: a group house splits by bed and night before it splits by person, which handles the couple who took the master suite; shared groceries have their own category logic; and whoever fronts the deposit is carrying a real cost until everyone settles up. Household weighting sits on top of those, not instead of them.
FAQ
Splitting trip costs with kids — quick answers
Straight answers on child shares, the published scales, and the cases where the rule actually matters.
01 Should kids count as half an adult when splitting a vacation rental?
It's a defensible rule, and closer to the published research than most people realize. The US Census Bureau's Supplemental Poverty Measure enters each child at 0.5 of an adult inside its equivalence formula — as an input, before a non-linear exponent that then discounts the whole household, and with a different figure (0.8) for a single parent's first child. The OECD-modified scale used across Europe is stingier, at 0.3 for children under 14, while the older Oxford scale also uses 0.5. On an illustrative $3,200 house shared by a family of four and a childless couple, the gap between the most and least family-friendly of these scales is about $97 — roughly 3% of the total.
02 What is an equivalence scale?
It's a formula that converts a household of any size and composition into a number of 'equivalent adults,' so households of different shapes can be compared on the same footing. Statistical agencies use them to measure poverty and income inequality. They exist because of economies of scale in consumption: as the US Bureau of Labor Statistics puts it, a family of two needs more than a single person but not double, because they share.
03 Which equivalence scale is the correct one?
There isn't one. The OECD's own methodology paper states that equivalence scales are to some extent conventional rather than derived from consumption analysis, that there is no universally accepted method for determining them, and that the OECD recommends no equivalence scale for general use. Deaton and Muellbauer's 1986 study found that the two classical estimation methods disagree by up to a factor of four on what children cost. Pick one, say which, and move on.
04 What does the folk 'adults = 1, kids = 0.5' rule get wrong?
The second adult, not the child. Every published scale discounts additional adults — 0.5 under the OECD-modified scale, 0.7 under the Oxford scale, and implicitly under the square root and Census SPM formulas — because people who live together share. The folk rule charges every adult a full unit. When every household is the same size this cancels out, but when sizes differ it overcharges the larger households and undercharges the smallest. On an illustrative $3,200 house split between three couples and one solo traveler, the solo traveler's share runs 14.3% under the folk rule versus 19.1% under the square root scale — a difference of about a third of their own bill.
05 Why not just itemize everything instead of using shares?
Do itemize wherever you can — a restaurant bill with line items needs no weighting theory, just a careful reading of the receipt. Weighting only becomes necessary for indivisible costs: one house, one rental car, one cleaning fee, one week of shared groceries. There is no per-person meter on a living room, so the only available lever is how much each person counts.
06 How do you weight a single parent's household?
The Census SPM scale is the only one of the four with an explicit answer: single-parent families use (adults + 0.8 × first child + 0.5 × other children) raised to the 0.7 power, treating the first child of a single parent as 0.8 of an adult rather than 0.5. The Census technical documentation notes only that the scale 'allows for a different adjustment for single parents,' citing Betson (1996); it does not spell out a rationale, so treat the 0.8 as a published parameter rather than a derived one.
