Short answer: a category split that starts even can drift away from even. “You take groceries, I take utilities” fixes who pays which bills. It says nothing about how many dollars each person pays. Groceries, electricity, internet and insurance do not get more expensive at the same rate. So the bucket with the faster-rising bills slowly takes a bigger share of the total, and nobody has to change anything for that to happen. The gap can also narrow or reverse when the fast movers change.
The national numbers show how wide the gap can get. Between September 2020 and September 2026, the Consumer Price Index for electricity rose 41.9%. Groceries rose 28.0%. Internet service rose 10.6%. The phone-plan index, which adjusts for what each plan includes, fell 1.8%. A split that put groceries on one side and electricity on the other would have moved apart even with both people paying every bill on time.
These are national averages. The Bureau of Labor Statistics says plainly that the CPI “does not necessarily measure your own experience with price change.” Your own bills will not follow these indexes exactly. Use them to see that categories move apart, then check the drift with your own statements.
Sources: Consumer Price Index for All Urban Consumers (CPI-U), U.S. Bureau of Labor Statistics, not seasonally adjusted, retrieved through the BLS public data API on 2026-10-18; Consumer Price Index Frequently Asked Questions, U.S. Bureau of Labor Statistics. Percent changes are our arithmetic on the published index values.
What does splitting bills by category mean?
Each person takes ownership of whole bills rather than a share of every bill. One partner pays rent; the other pays groceries and utilities. One roommate covers internet and the phone plan; the other covers electricity and gas. SupportPay, which builds tools for shared family costs, describes the same setup among siblings caring for a parent: “One brother pays for all of the medical care. One sister pays for groceries and utilities.” It adds the caveat that matters here: “you need to keep a close eye on things to make sure the load stays balanced over time.”
The appeal is real. Nobody has to send a payment every month, each person deals with their own providers, and there is no shared account to manage. Researchers who study how couples handle money call arrangements where each partner keeps their own money independent management. Katherine Ashby and Carole Burgoyne describe it as relatively under-researched but increasingly popular. Their interviews with cohabiting and engaged couples found that the label told you little about each partner’s actual standard of living or access to money. Couples using it handled money “in a wide variety of ways.”
That is the problem in one sentence. “We each pay our own bills” describes the setup. It does not tell you whether the load is still even.
Sources: When Caring For an Older Adult Turns Into a Second Job, SupportPay, 2026-04-10; Ashby & Burgoyne, Separate financial entities? Beyond categories of money management, The Journal of Socio-Economics, 2008
Why does a category split drift?
Because each bucket follows its own prices. The table shows the change in the national price index for common household categories over six years. The spread runs from a 1.8% fall to a 56.5% rise.
One caution on phone plans. BLS says the wireless index “uses prices estimated from hedonic regression models to control for quality change.” So a plan that costs the same but includes more can count as a price fall. The -1.8% does not necessarily mean phone bills got smaller in dollars.
Why does the drift go unnoticed? Our reading is that the setup never asks anyone to compare the buckets. Richard Thaler’s review of mental accounting describes how people group spending into categories such as housing and food, and how accounts “can be balanced daily, weekly, yearly, and so on.” In a category split, each person balances their own account. Nothing in the arrangement prompts a check of one account against the other.
This is the same slow-creep pattern behind roommate debts that grow without anyone noticing, with one difference: nobody is cutting corners. The prices moved, and the agreement stayed where it was.
Sources: CPI-U category indexes, U.S. Bureau of Labor Statistics, U.S. city average, not seasonally adjusted, September values (our arithmetic); Use of alternative data and methods in the CPI for wireless telephone services, U.S. Bureau of Labor Statistics; Thaler, Mental accounting matters, Journal of Behavioral Decision Making, 1999
How far can a 50/50 category split drift?
Far enough to matter, when each bucket holds only a few kinds of bill. The three examples below start at an even split in September 2020. The starting dollar amounts are illustrative, and every one is listed under the table. Each bill is then grown by its category’s national index to September 2026, so the examples show what the price changes alone would do.
Starting amounts a month. Siblings: medical care $600; groceries $450 and electricity $150. Roommates: groceries $200; electricity $140 and gas $60. Couple: rent $1,500; groceries $1,000, electricity $150, gas $60, water, sewer and trash $90, and car insurance $200. The examples leave out the phone-plan and internet indexes, so the quality adjustment in the phone-plan index does not affect any dollar figure.
The sibling example uses the split SupportPay describes. The sibling who took medical care started with $600 a month. At the national medical index, up 13.6% over the six years, that bucket grows to $681.67. The other sibling’s $450 of groceries and $150 of electricity grow by 28.0% and 41.9%, to $788.83 together.
One caution on the medical index. BLS says the medical care indexes measure “the total reimbursement to medical care providers,” including payments by private insurers and Medicare. What a family pays out of pocket for a parent’s care can move differently, which is one more reason to check your own statements.
Nobody made a bad decision. The split was even when they agreed it, and the gap came entirely from prices.
Sources: CPI-U category indexes, U.S. Bureau of Labor Statistics, September 2020 and September 2026 values; Measuring price change in the CPI: Medical care, U.S. Bureau of Labor Statistics. Worked examples are our arithmetic on illustrative starting amounts.
Why do some category splits drift less?
Because the biggest bill on each side sets most of the trend. In the couple’s example, rent faced a bucket where groceries were two thirds of the money, and those two indexes rose at similar rates: 31.3% and 28.0%. Electricity, gas and car insurance rose much faster, but together they started at $410 of the $1,500. So the couple moved only to 49.4% vs 50.6%, a gap of $46.67 a month.
Treat this as one illustration with one set of amounts. The national numbers do not show that more categories always mean less drift. Two bills can offset each other, and a bucket of many bills can still drift if its largest bill moves fast. The check that matters is whether the biggest bills on the two sides follow similar trends. If they do not, plan to check the split more often.
How should you divide bills into buckets?
Build each bucket so its price risk is spread out. Four rules follow from the national numbers above.
In the six years to September 2026, energy and car insurance rose fastest, and internet and phone plans moved least. Those rankings may not repeat, but a bucket that holds only one of those groups takes on that group’s trend, whatever it turns out to be.
The largest bill carries most of a bucket’s weight. In the couple’s example, rent faced a bucket led by groceries, and their indexes rose 31.3% and 28.0%. If the two biggest bills follow very different trends, expect more drift.
If a bill in a bucket is paid once or twice a year, such as an insurance premium, divide it into a monthly figure before you compare the two buckets.
Agree on the share each bucket should carry, such as 50/50 or a split by income, when you set it up. Without a written target, there is nothing to check against later.
Can you build the drift in up front?
Not reliably. The fast movers change from year to year, and some of them reverse. Looking at the national index from one September to the next:
Piped gas rose 33.1% from September 2021 to September 2022, then fell 19.9% the next year. Car insurance rose 16.3% from September 2023 to September 2024, rose 3.1% the next year, then fell 5.0% from September 2025 to September 2026. A fixed adjustment tuned to one year’s trend would soon have pointed the wrong way. Chip Heath and Jack Soll’s research on mental budgeting describes a similar limit for one person’s budget: because budgets “cannot perfectly anticipate consumption opportunities,” people may earmark “too much or too little money for a particular category.” Their studies were about individual budgets, not shared bills. But a category split is a budget two people set together, and by analogy it faces the same limit. The fix is to look back at what was actually paid, on a schedule, rather than guess ahead.
Sources: CPI-U category indexes, U.S. Bureau of Labor Statistics, September-to-September changes (our arithmetic); Heath & Soll, Mental Budgeting and Consumer Decisions, Journal of Consumer Research, 1996
How do you true up a category split?
Once a year, total what each person actually paid and compare it with the share you agreed. It takes one sitting with your bank or card statements.
Lease renewal or the first week of January both work. A date you set in advance turns the check into a routine that nobody has to raise.
Add up the last 12 months of each person’s bills. Use the real amounts on your statements. Twelve months also smooths out seasonal bills such as winter heating.
The target does not have to be 50/50. Morton Deutsch’s work on distributive justice separates three bases for dividing things in a group: equity, equality and need. On our reading, an income-based split applies an equity-style rule. Decide which one you are using before you do the arithmetic.
First settle the year that just ended. Work out each person’s target: their agreed share times the combined total. Whoever paid less than their target sends the difference to the other person. On a 50/50 target, that is half the gap between the two totals. Then fix the year ahead: move a single bill from the heavier bucket to the lighter one, choosing one whose cost is close to the shift you need so you do not overshoot, or take the fastest-moving category out of the buckets and split that one by amount every month.
For bills that change month to month, such as energy, the methods in how to split utility bills fairly apply. If you are setting up a household from scratch, the new-roommate expense setup guide walks through each category. Couples weighing equal against income-based shares can start with proportional, equal and pooled systems.
Source: Deutsch, Equity, Equality, and Need, Journal of Social Issues, 1975
Where splitty fits
A category split covers the recurring bills. It does not cover the shared one-off receipt, such as a grocery run where half the cart is shared and half is one person’s. That is where splitty fits. Scan the itemized receipt. Every item starts split among everyone, so leave the shared items as they are and tap to remove anyone who didn’t share an item. Then send each person a pre-filled payment request.
splitty settles a bill now. It does not keep a running balance over months. For a household ledger that tracks who owes what across the year, Splitwise is built for that job. Splitwise tracks; splitty settles.
Common questions
FAQ
Questions & Answers
01 Is splitting bills by category fair?
It can be fair on the day you set it. Over time it tends to drift, because each category's prices rise at a different rate. In the national CPI, electricity rose 41.9% from September 2020 to September 2026 while internet service rose 10.6%. Check it once a year against what each person actually paid.
02 How often should we rebalance a category split?
A yearly check on a fixed date is a simple default. Pick a date such as lease renewal, total the last 12 months of each person's bills from your statements, and compare the totals with the share you agreed.
03 Which household bills have risen fastest?
In the national CPI from September 2020 to September 2026, car insurance rose 56.5%, piped gas 53.7% and electricity 41.9%. Rent rose 31.3% and groceries 28.0%. Medical care rose 13.6%, internet service 10.6%, and the quality-adjusted phone-plan index fell 1.8%. Your own bills may differ from these averages.
04 What is the simplest way to fix a drifted split?
Settle the past year with one catch-up payment. Whoever paid less than their agreed share of the combined total sends the difference; on a 50/50 split, that is half the gap between the two totals. Then, for the year ahead, move one bill from the heavier bucket to the lighter one, or take the most volatile bill out of the buckets and split it by amount each month.
05 Should we use the CPI to adjust our split?
No. Use your own bills. The Bureau of Labor Statistics says the CPI does not necessarily measure your own experience with price change. The national index shows that categories move apart. Your statements show what you owe.
06 Does a category split work for siblings sharing a parent's costs?
SupportPay describes it as one option for siblings, for example one sibling paying medical care and another paying groceries and utilities. The same drift applies. In an illustrative example at national rates, a $600 vs $600 split of that kind moved to 46.4% vs 53.6% over six years. A yearly true-up keeps it even.
