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 = total / roommates square_foot_split = total × (your_room_sqft / total_room_sqft) income_split = total × (your_income / total_income)

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):

• Big room: $3,400 × (200/450) = $1,511
• Medium room: $3,400 × (150/450) = $1,133
• Small room: $3,400 × (100/450) = $756

The big-room person pays 33% more, the small-room person pays 33% less.
This is what most roommate-fairness calculators recommend, and 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.
It produces the same dollar amounts as the square-foot model in this example by coincidence — usually they differ.

The hybrid model.
Many households split rent by room size and utilities equally.
The logic: room size determines housing benefit, but everyone uses similar amounts of electricity, water, and internet.
This is probably the most common setup in long-term roommate situations and is easier to explain when someone moves out and a new roommate arrives.

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.