Roommate Utility Split Calculator

Splitting bills between roommates causes more friction than almost any other household issue.
Equal split is the simplest method but is not always the fairest. The person with the master bedroom and en-suite uses more space than the person in the small back bedroom.
The math depends on which fairness model you pick.

The three common methods:

equal_split = (rent + utilities) / roommates square_foot_split = rent × (your_room_sqft / total_room_sqft) + utilities / roommates income_split = (rent + utilities) × (your_income / total_income)

Note what the middle one does: it divides the rent by room size but the utilities equally. Room size decides how much housing you get. It does not decide how much electricity you use.

A worked example.
3 roommates in a $3,000/month apartment with $400/month in shared utilities.
Total monthly: $3,400.

Equal split: $1,133 each.
That works if rooms are similar sizes and incomes are similar.

Square-foot split with rooms of 200, 150, and 100 sq ft (total 450). Rent by room size, then $400 of utilities divided three ways at $133.33 each:

• Big room: $3,000 × (200/450) + $133.33 = $1,466.67
• Medium room: $3,000 × (150/450) + $133.33 = $1,133.33
• Small room: $3,000 × (100/450) + $133.33 = $800.00

The big-room person pays about 29% more than an equal split, the small-room person about 29% less.
This is what most leases for shared apartments default to.

Income-weighted split is less common.
3 roommates earning $80K, $60K, and $40K (total $180K):

• High earner: $3,400 × (80/180) = $1,511
• Mid earner: $3,400 × (60/180) = $1,133
• Low earner: $3,400 × (40/180) = $756

This is the model used by some college roommate situations and intentional communities.
Notice it lands somewhere different from the square-foot split above: $1,511 against $1,467 for the top payer, $756 against $800 for the bottom one. Income weighting spreads the gap wider, because it applies to the utilities as well as the rent.

That hybrid, rent by room size with utilities split equally, is the one this calculator uses for the square-foot option, and it is probably the most common setup in long-term roommate situations. It is also the easiest to explain when someone moves out and a new roommate arrives, because only the rent share has to be renegotiated.

A few practical points.
Pick the method before signing the lease, not after the first bill arrives.
Document the agreement in writing, even between close friends.
Apps like Splitwise (free, well-designed) handle the running tally automatically and remove the awkward monthly Venmo conversation.
For utilities specifically, take 12-month averages where possible. Winter heating and summer AC can be very different.

A note on fairness when one roommate has a partner sleeping over often.
Splitting utilities by “person-nights” rather than headcount is sometimes proposed but rarely implemented.
A more practical approach: if a partner stays 4+ nights a week consistently, they should pay a reduced share of utilities (often $100-200/month) or contribute to grocery costs.
Establishing this norm early prevents the resentment that builds quietly over months of unequal use.