Splitting Bills by Category: Why the Split Drifts Over Time

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.

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.

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