70/30 split bills: when it works (UK)

A 70/30 bills split is a simple, stable rule: one person pays 70% of shared costs, the other pays 30%. It can feel fair when one income is meaningfully higher, but it only stays fair if the leftovers (and stress tests) still work for both people.

Last updated: 2025-12-17

Built by Brandon

Methodology

Use the calculator with a 70/30 split

Opens the main calculator prefilled so you can swap in your numbers and compare 70/30 vs proportional vs 50/50.

No signup. Private. Links are shareable and can be reset anytime.

Use the calculator with these defaults Open the calculator

How 70/30 compares to proportional

Proportional splitting is a moving target: it follows your take‑home incomes. If A earns 70% of the total, then proportional and 70/30 are basically the same. If A earns 55% and you still do 70/30, you’re choosing a different outcome on purpose — usually to protect one person’s savings, account for a deposit difference, or keep admin simple.

The practical test is not the label. Run your real numbers, check “leftover each”, then stress test rate rises (+1%, +2%) and a one‑income month. Budgeting guidance only — not financial advice.

Worked examples (with real numbers)

Each example shows what 70/30 does versus proportional and 50/50. Open a scenario to tweak it.

Example 1: 70/30 is close to proportional

Incomes: A £4,200 / B £1,800 · Home £425,000 · Deposit 12%

Total shared cost: £2,953 / month

Proportional

A £2,067 · B £886

50 / 50

A £1,477 · B £1,477

Selected: 70 / 30 · Leftovers: A £2,133 / B £914

Open this scenario in the calculator Compare 66/33 vs 50/50

Example 2: 70/30 protects one person’s savings

Incomes: A £3,300 / B £2,900 · Home £480,000 · Deposit £75,000

Total shared cost: £3,363 / month

Proportional

A £1,790 · B £1,573

50 / 50

A £1,682 · B £1,682

Selected: 70 / 30 · Leftovers: A £946 / B £1,891

Open this scenario in the calculator Compare 66/33 vs 50/50

A calm way to agree 70/30

Decide what “shared” means (mortgage, council tax, utilities, insurance, maintenance buffer, and optionally groceries).
Run 70/30 and proportional. If one version leaves someone with too little margin, adjust (custom %, groceries toggle, or home price).
Stress test it. If one month on one income would break you, plan an emergency fund or reduce commitments.
Set a review trigger and write down the rule you agreed (so it doesn’t become a monthly debate).

FAQ

Is 70/30 the same as splitting by income?

Only if your take‑home incomes are roughly a 70/30 split. Proportional adjusts automatically; 70/30 is a fixed rule that can drift if salaries change.

Should 70/30 apply to everything?

Not always. Some couples do mortgage proportional but split certain bills 50/50 (or vice‑versa). Decide what’s shared, then sanity‑check leftovers and stress tests.

What if one person paid most of the deposit?

A bigger deposit affects ownership and risk. Many couples treat the deposit separately (tracked as equity) and still split monthly costs by income — or agree a custom split that reflects both affordability and contribution.

How do we keep it fair over time?

Pick a review rule. For example: revisit when either income changes by £300+/month, or every 6 months, and rerun the stress tests (+1%, +2%, one‑income).