Splitting Bills by Days of Occupancy: Partial Months

A flatmate moves in on the 15th. Another leaves on the 20th. A third is on a research exchange and only sleeps in the apartment ten nights a month. The electricity bill arrives covering the full thirty days. Equal split says everyone owes a third — but two of the three flatmates were barely there. What is the fair number?

Splitting utility bills by days of occupancy means each flatmate pays in proportion to the number of days they were actually present during the billing period, so partial months, mid-month moves, and frequent absences all get reflected in the math automatically. It is the third of the four common methods, and the only one designed from the start to handle the messy reality of high-turnover flats. This article goes deep on when per-day is the right call, the math that survives an argument, how to track presence without making the flat feel surveilled, and the edge cases that come up in practice.

Quick context first: this is a deep-dive companion to a broader piece comparing all four splitting methods side by side. If you have not picked a method yet, start with Splitting utility bills among flatmates: 4 methods compared and come back here for the per-day details.

When per-day occupancy is the right method

Per-day splits work because some utility consumption genuinely tracks with bodies-in-the-flat. Electricity, gas, water, and heating all scale with the number of people actively using the apartment. When occupancy varies meaningfully across the month, equal split overcharges the people who were not there and undercharges the people who were.

It is the right call when at least one of the following is true:

• High turnover. Co-living setups, short-term arrangements, student housing where flatmates arrive and leave mid-term. The flat where the roster looks different every month is where equal split starts losing legitimacy.
• Exchange students, interns, or fixed-term programs with non-aligned start dates. A flatmate starting an internship on the 10th of October and another finishing their semester on the 22nd of December will hit partial months on both ends. Per-day handles both automatically.
• Frequent business travel. One flatmate gone Monday-to-Thursday every week is in the apartment less than half the time — a 40-60% difference in occupancy that equal split ignores.
• Sabbaticals, parental leave, or temporary moves. A flatmate visiting family abroad for three weeks should not pay the full month while the others enjoy a quieter, cheaper flat.
• Anyone moving in or out mid-month. Someone arriving on the 15th has consumed half the utilities; charging them the full month is indefensible.

The combination that makes per-day clearly the right call: at least one person’s occupancy varies by more than a week per month, the bills are large enough that the variance matters, and the flatmates value precision over simplicity.

When per-day occupancy is the wrong method

Per-day adds tracking overhead that only earns its keep when occupancy actually varies. Forcing it onto a stable flat creates admin without fairness gains.

The clearest mismatches:

• Stable, long-term flats. Three flatmates who all live in the apartment every day do not need per-day tracking. The result is identical to equal split.
• Bills that do not scale with occupancy. Basic internet, condo dues, building insurance, any fixed monthly fee. Splitting them per-day produces the same number but with extra steps.
• Groups that do not want to track presence. Some flats feel uncomfortable logging who slept where. Forcing per-day there creates resentment that outweighs the fairness gain.
• Very short billing cycles. Daily tracking on weekly or ten-day cycles gets exhausting fast. The lighter the cycle, the higher the overhead-to-fairness ratio.

A useful test: if every flatmate’s presence count would be roughly the same (within two or three days), per-day produces nearly the same result as equal split and is not worth the tracking.

How to track presence without making the flat feel surveilled

The math is only as fair as the tracking. Most flatmate disputes about per-day splits trace back to the question “wait, were you actually here on the 12th?” — not to the formula itself.

A few approaches that work, in increasing order of formality:

• Shared calendar. A Google Calendar shared among the flatmates where everyone marks absences in advance. Low friction, opt-in, permanent record without anyone feeling watched.
• Presence log in the flat chat. A pinned message updated whenever someone is away (“I’ll be in Berlin from the 14th to the 21st”). Equally evidence-grade if everyone respects it.
• App-based tracking. A dedicated tool where each flatmate logs their days. Plinthos handles per-day splits as one of the four built-in methods — you mark absences in the app and the bill calculation factors them in automatically.
• Honor system with occasional reconciliation. Everyone tracks loosely, and at bill time the flatmates compare notes and agree on a count. Works in trusting flats; falls apart the first time memories diverge.
• Written notice for longer absences. Anything more than two or three days should be flagged in advance, in writing.

The common factor: the record exists before the bill, not after. Retroactive reconstruction is where arguments start. A flatmate who says “I was definitely away that week” three weeks later, with no chat message and no calendar entry, is asking everyone else to take their word for it.

The math, with worked examples

The formula is simpler than it looks. The bill is divided by the total number of person-days across all flatmates, and each flatmate’s share is their personal days multiplied by that per-day rate.

Bill total divided by total person-days, multiplied by each person’s days, equals their share.

Two worked examples with three flatmates and a thirty-day billing period.

Scenario A: one flatmate away for ten days. Bill of 240, one flatmate present 20 days, the other two present all 30.

• Total person-days: 30 + 30 + 20 = 80
• Per-day rate: 240 divided by 80 = 3.00
• The two who were present: 30 times 3.00 = 90 each
• The one who was away: 20 times 3.00 = 60

Total: 90 + 90 + 60 = 240. The absent flatmate saves 20 compared to equal split; the present ones each pay 10 more. That is the redistribution the method is designed to produce. If instead everyone had been present all 30 days, each person-days share would land at 80 — identical to equal split, which is exactly what should happen when occupancy is uniform.

Scenario B: a new flatmate arrives mid-month. A fourth flatmate moves in on the 15th, so they are present for 15 days. The other three are present all 30 days. Same bill of 240.

• Total person-days: 30 + 30 + 30 + 15 = 105
• Per-day rate: 240 divided by 105 = 2.286
• The three full-month flatmates: 30 times 2.286 = 68.57 each
• The new flatmate: 15 times 2.286 = 34.29

Total: 68.57 + 68.57 + 68.57 + 34.29 = 240. The new flatmate’s share reflects exactly the half-month they actually lived in the apartment.

The pattern is always the same: bill divided by total person-days produces a daily rate; each flatmate’s share is their days multiplied by that rate. The arithmetic survives any combination of moves, travels, and partial stays.

Edge cases that come up in real tenancies

Most apartments running per-day splits will hit at least one of these during a normal year.

• Common-area dependent bills. Water and winter heating both scale with occupancy because the people who are there use the water and warm the rooms. The exception is background heat during a cold spell when the flat needs warming even with nobody home — handle that with a small floor amount split equally before per-day kicks in.
• Fixed-fee bills. Netflix, gym memberships, condo dues, basic internet. These do not scale with presence. Switch to equal split for those specifically — the small costs that wreck the bill math guide covers the dozen recurring micro-expenses that need their own convention.
• Partial-day arrivals and departures. Common convention: arrivals count from the next day, departures count through the departure day. Pick the rule once, write it down.
• Disputed days. Pre-agreed rules help: who-was-present defaults to “present” unless there is a calendar entry or chat message marking the absence. Benefit of the doubt avoids endless reconstruction.
• Guest stays. A flatmate’s partner sleeping over three nights a week consumes utilities the bill is reflecting. Some flats add a per-night guest surcharge for long stays; others increase the host’s effective person-days. Set the rule before the guest moves in.

Per-day is a good default for variable occupancy, not a sacred rule. When the assumption breaks, the method needs to adjust.

The hybrid pattern most variable-occupancy flats end up using

After a few months, most flats with meaningful turnover converge on a hybrid that uses per-day only for the utilities where it actually moves the number. The reasoning behind matching method to utility — rather than forcing one rule onto everything — is what fair vs equal bill splitting calls distributive fairness.

A pattern that works in practice:

• Electricity, gas, water, heating: per day of occupancy (with a small per-square-meter component on heating if room sizes differ a lot).
• Internet: equal split. Bandwidth does not scale with bodies in the flat.
• Streaming and subscriptions: equal split, or excluded if personal.
• Cleaning, condo dues, building insurance: equal split.

This gives most of the fairness benefit with about half the admin. The flatmate who travels every other week pays less for the gas and water bills they did not consume, and the same as everyone else for the Netflix subscription that ran whether they were home or not.

If you want the daily-tracking and bill math to disappear, Plinthos splits utility bills by day of occupancy as one of its four built-in methods, alongside equal, per square meter, and custom. You mark absences in the app, upload the bill (or use OCR to extract the amount), and the per-day calculation applies automatically — every flatmate sees the original document next to their own share. See how it works → /en/#how-it-works.

Common mistakes when applying the method

These come up in almost every flat that tries per-day for the first time.

• Forgetting to track until the bill arrives. Retroactive reconstruction is the single biggest source of disputes. Track in real time — even a chat message is enough — and the bill conversation becomes mechanical.
• Not counting partial days. Move-in day and move-out day exist. Pick a rule and apply it consistently. Pretending those days are zero or full creates predictable arguments.
• Punishing absence. A flatmate who is away half the month is not “skipping their share.” They are consuming less. The redistribution is the method working correctly, not a penalty being collected.
• Mixing methods on the same bill across months without consent. Per-day in January, equal in February, per-day again in March because it suits whoever does the math — that is how trust gets lost. Pick a method per utility and stick to it.
• Tracking presence in a way that feels invasive. Heavy logging, group polls, “where were you last Tuesday?” — none of that. Lightweight calendar entries, opt-in absence notices, and trust on the rest.

Done right, per-day is invisible — flatmates note absences in advance, the bill arrives, the math is mechanical. Done wrong, it turns into a monthly forensic exercise. The difference is whether the tracking happens before or after the bill.

When to switch methods

Per-day is not a forever choice. Signals that it is time to reconsider:

• Admin has become too much. If tracking is taking real time every month and the result is barely different from equal split, roll back to equal.
• Unfairness becomes obvious for one specific bill. If heating dominates because winter arrived and one flatmate works from home all day, switch that bill to a hybrid (per-day plus per-square-meter) and leave the others on per-day.
• The flat composition changes. A new flatmate who lives there full-time when the previous one was always traveling means the variance just dropped.
• Someone keeps disputing the days. That is a trust problem, not a per-day problem. Either fix the tracking or switch to a simpler method.

The switch always happens with everyone’s written consent and going forward only. The new method applies from a specific bill onward — even a chat message confirming “agreed, switching to equal split from the November bill” is enough as evidence.

A related read worth keeping handy when the bill conversations get tense: Flatmate conflict mediation covers how to handle the discussions that come up around money specifically.

Frequently asked questions

How do you count the day someone moves in or moves out?

Pick a convention before the first move. The most common one: arrival day is not counted (the new flatmate starts paying from the day after move-in), and departure day is counted in full. This avoids double-counting on swap days. Whatever you pick, apply it consistently — the unfairness comes from inconsistency, not from the specific rule.

Does per-day work for water and heating, or just electricity?

It works for any utility whose consumption scales with people-in-the-flat. Water and gas both qualify — fewer people means fewer showers, less cooking, less heat being asked of the boiler. The exception is background heating during cold weather, where the flat needs a baseline of warmth even with nobody home. Handle that by splitting a fixed baseline portion equally and applying per-day to the rest.

What if one flatmate refuses to share their schedule?

Per-day requires consent and minimal transparency about absences. If a flatmate will not share the rough shape of their month, per-day is not viable — switch to equal split or per-square-meter. The method depends on a shared tracking record; without it, the math cannot be defended.

Can we use per-day for some bills and equal for others?

Yes, and most variable-occupancy flats end up doing exactly this. Per-day for electricity, gas, water, and heating; equal split for internet, subscriptions, cleaning, and any condo dues. The rule sheet specifies which method applies to which bill, signed once at handover.

How do we handle a flatmate present but working from home all day?

Per-day undercounts that flatmate’s consumption — they are heating, lighting, and powering a flat the others have left for the office. The fix is a hybrid: per-day baseline plus a small work-from-home surcharge. Negotiate it openly rather than letting resentment build.

Closing

Per-day occupancy splits are the right answer when the people in the flat are not the same people every day. They handle partial months, exchanges, business travel, and mid-month moves with arithmetic that survives an argument — as long as the tracking happens in real time and not after the bill arrives. They are overkill when occupancy is stable, and they are the wrong method for utilities that do not scale with bodies in the flat.

Pick the method per utility, agree the tracking convention on day one, write it down, and the rest is mechanical.

If you want the tracking and bill math to disappear, Plinthos splits utility bills by day of occupancy among the four built-in methods, keeps a record of marked absences alongside the original bill, and shows every flatmate exactly how their share was calculated. See features →

This article describes general practices for splitting utility bills by days of occupancy. Local laws may impose specific requirements on tenancy agreements and shared housing — when in doubt, check your local rental rules or consult a tenant association.