It starts with a few weekends. Then it is Thursday through Sunday. Nobody signed a lease with the partner, but the showers are longer, the laundry is bigger and the electricity bill is going up.
You don't have to turn this into a fight. Count the nights, then split what is actually used.
This is a practical guide, not legal advice. Who may live in a home depends on your lease, your landlord and where you live. Check both before you add anyone.
Start with a guest rule
Write down, once, what "guest" means. For example: up to a few nights a week is a guest, and beyond that we talk. The exact number is yours to pick. The point is to agree on the line before anyone is over it.
Split by what the partner actually uses
Fixed bills. Internet and rent cost the same whether the partner is there or not. The tenants split these as they already do.
Usage bills. Electricity, water and gas rise with another person. Count the partner's nights and add them to the person-days, the same method as for a roommate who is away.
Shared supplies. If the partner uses the toilet paper and the kitchen, add them to the household supplies, or have the host chip in a small amount.
Who pays the partner's share?
The simplest answer is the roommate who invited them. They are the one who agreed to the extra costs, so the partner's share goes on their side of the ledger. Whether they pass it on to their partner is their business.
Worked example (illustrative numbers)
Three roommates share a flat, 30-day month. Cam's partner stayed over for 10 nights.
- Internet: $60 (fixed)
- Electricity: $120 (variable)
- Water: $45 (variable)
Person-days: Ana 30, Ben 30, Cam 30, partner 10 = 100.
Internet: split equally among the three tenants: $20 each.
Electricity: $120 / 100 = $1.20 per day. Ana $36, Ben $36, Cam $36, partner $12.
Water: $45 / 100 = $0.45 per day. Ana $13.50, Ben $13.50, Cam $13.50, partner $4.50.
Cam covers the partner's $12 + $4.50 = $16.50.
| Internet | Electricity | Water | Total | |
|---|---|---|---|---|
| Ana | $20 | $36 | $13.50 | $69.50 |
| Ben | $20 | $36 | $13.50 | $69.50 |
| Cam (incl. partner) | $20 | $48 | $18 | $86 |
The three totals add up to $225, which is exactly the three bills combined. These numbers are made up; swap in your own.
What about rent?
Rent is a different conversation. If the partner is effectively living there, the tenants may want a rent contribution, and the lease and landlord may have a say in whether an extra resident is allowed. Raise it early and calmly, and let the roommate whose partner it is bring it up first.
Do the math faster
The utility bill splitter takes each person's days at home. Add the partner as a fourth person and assign their share to the host. For rent between tenants, see the rent splitter. Both free, no signup.
FAQ
How many nights makes someone a guest? There is no universal number. Pick a line that all of you accept, write it down and revisit it when things change.
Should the partner chip in for groceries? If they eat from the shared food, yes, a share of the shared pile or a small fixed amount. Personal food stays personal.
What if a roommate refuses to talk about it? Start with the bill. "Electricity went up by this much after the nights we counted" is easier to discuss than "your partner is always here".
Keep it friendly, keep it written
Billmates is a web app, still on the waitlist, that tracks who owes what in a shared home. It never holds or moves your money. You pay each other the way you already do. Roommates join from a browser link. No ads. Leave your email to hear when it's ready.