Buy Now Pay Later Calculator — the United Kingdom 2026-27
What a buy-now-pay-later plan costs once instalment fees, a missed payment and the late-fee cap are counted — and what the same purchase would have cost on a credit card.
A four-payment plan divides the purchase by four and takes the first instalment at checkout, so the arithmetic on the balance is trivial and the cost is entirely in the fees.
| Principal · #1 | £125.00 |
|---|---|
| Principal · #2 | £125.00 |
| Principal · #3 | £125.00 |
| Principal · #4 | £125.00 |
| Fee · #1 | £0.00 |
| Fee · #2 | £0.00 |
| Fee · #3 | £0.00 |
| Fee · #4 | £0.00 |
| Late fee · #1 | £0.00 |
| Late fee · #2 | £0.00 |
| Late fee · #3 | £0.00 |
| Late fee · #4 | £0.00 |
| BNPL on time | £0.00 |
|---|---|
| BNPL with fees | £0.00 |
| Credit card 20% | £15.38 |
How the buy now pay later works
A four-payment plan divides the purchase by four and takes the first instalment at checkout, so the arithmetic on the balance is trivial and the cost is entirely in the fees. That is the part worth measuring. An instalment fee is charged per payment, not per order, so a fee of ten on a four-payment plan is forty — and expressed as an annual rate over the eight weeks the plan actually runs, a small flat fee becomes a large percentage, which is why the effective annual rate here is usually the number that surprises people. Late fees are capped by law or by policy rather than accumulating: the cap is the lesser of a per-instalment amount and a proportion of the order, and once you have hit it, further missed payments cost nothing more in fees — though they still cost you the account. The credit-card comparison holds the same purchase over the same weeks at a card rate, which is the honest comparison because both are ways of not paying today; it is not a comparison against paying cash, which costs nothing at all.
Formula: per payment = amount ÷ instalments + fee per instalment · late fees = min(missed × fee, cap) · effective annual rate = (total extra ÷ amount) × (52 ÷ weeks)
Where these figures come from
Debt and credit figures on this page come from the Financial Conduct Authority (consumer credit rules), The Bank of England (rate data), and MoneyHelper — the UK's government-backed money-guidance service operated by the Money and Pensions Service (MaPS).
- Consumer credit rules — FCA — Consumer credit.
- Bank Rate (affects variable-rate debt) — Bank of England — Bank Rate.
- Debt help & free advice — MoneyHelper — Dealing with debt.
- Student loan repayment — GOV.UK — Repaying your student loan.
Last checked: April 2026. Rates and thresholds are reviewed against the source of record each November, when annual adjustments for the following tax year are published.
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Afterpay — £500 purchase · 4 fortnightly payments £125/fortnight · free if on time · £10 late fee cap
Afterpay splits a £500 purchase into 4 fortnightly payments of £125. If paid on time: no interest, no fees — effectively free. One missed payment: £10 late fee (capped at 25% of purchase). Total with one late fee: £510.
| Purchase amount | £500 |
|---|---|
| Each payment (×4 fortnightly) | £125 |
| Total if paid on time | £500 (no extra cost) |
| First late fee | £10 |
| Cap on late fees (Afterpay) | 25% of order value = £125 max |
| Effective rate on time | 0% — genuinely free |
- If you miss all 4 payments £125 in late fees = £625 total
- BNPL vs credit card (20%) BNPL saves £17 in interest if on time
- Merchant fee (not your cost) Retailer pays 4–6% to Afterpay
- BNPL regulation (2025) Coming under FCA regulation — affordability checks
- Impact on credit file Missed payments may show on your credit file
Zip Pay in 4 — £1,000 purchase · 1% installment fee £255.10/payment × 4 · effective rate ~26% per year
A £1,000 purchase on Zip Pay in 4 with a 1% per-installment fee: each payment = £255 (£250 principal + £5 fee). Total paid: £1,020. The installment fee of £20 total equates to an annualised effective rate of approximately 26% — similar to a credit card.
| Purchase amount | £1,000 |
|---|---|
| Per payment (×4 fortnightly) | £255 (£250 + £5 fee) |
| Total cost | £1,020 (+£20 fees) |
| Total fees | £20 (1% × 4 payments) |
| Effective annualised rate | ~26% per year |
| Vs credit card at 20% | £1,015 total — BNPL costs more |
- If also a late fee +£10–£15 per missed payment
- Opportunity cost if using savings Lost £15 interest (3% savings rate)
- At £2,000 purchase £40 in fees (same 26% effective rate)
- Vs 0% credit card offer 0% card is cheaper if you qualify
- BNPL vs buy outright Buying outright always cheapest — budget first
Klarna Financing — £2,000 · 12 months · 19.99% per year £185/month · £220 total interest
Klarna offers longer-term financing in the United Kingdom at rates around 19.99% per year A £2,000 purchase over 12 months: monthly payment of approximately £184, total interest of approximately £208, total repaid £2,208. This is comparable to a personal loan or credit card.
| Purchase amount | £2,000 |
|---|---|
| Term | 12 months |
| Rate | 19.99% per year |
| Monthly payment | £184/month |
| Total interest | £208 |
| Total repaid | £2,208 |
- Vs personal loan (12% per year) Saves £80 in interest
- Vs credit card (20% per year) Similar cost — no advantage
- Vs Afterpay (if eligible) Afterpay on £2,000 is interest-free
- Early repayment Check Klarna terms — fees may apply
- Credit check required Longer-term Klarna requires hard credit check
BNPL vs credit card — true cost comparison BNPL wins if on time · card wins for larger/longer
BNPL is cheaper than a credit card IF you pay on time (no fees vs 20%+ interest). But if you miss payments, BNPL late fees and interest can exceed credit card costs for small purchases. For amounts over £1,000 or terms over 6 weeks, a 0% purchase credit card or personal loan may be cheaper.
| BNPL on time | £0 fees — genuinely free |
|---|---|
| BNPL with 1 late fee | £10–£15 extra (small purchase) |
| Credit card (20%) · 6 weeks | £12 interest on £500 |
| Credit card (0% offer) · 6 months | £0 interest if cleared in time |
| Personal loan (10%) · 12 months | £28 interest on £500 |
| BNPL with extended financing | 15–30% per year — same as credit card |
- Best option: have the cash Pay outright — zero cost
- Best free credit: 0% card offer 55–60 day interest-free period
- Best BNPL: pay on time Afterpay free if no missed payments
- Worst outcome BNPL + late fees + credit card interest
- FCA warning 23% of BNPL users cut back on essentials
Worked examples
| Inputs | Per Payment | Note |
|---|---|---|
| 400 over four payments, no fees, paid on time | £100.00 | 100 a fortnight and nothing on top |
| The same plan with a fee on every instalment | £106.00 | a 6 fee is 24 on the order, and 39% a year in equivalent terms |
| Two payments missed | £100.00 | the late fees stop at the cap, the account does not |
The sources behind this calculator
- CFA Institute — Quantitative Methods: The Time Value of Money — the discounting, annuity and IRR conventions used here
- US Federal Reserve — Regulation Z (Truth in Lending), APR calculation — why APR includes fees where a nominal rate does not
- MoneyHelper (UK Government-backed) — the UK's free money guidance service
Last checked: September 2026. These are the standard textbook formulas; the conventions named are those of the CFA Institute curriculum and ISO 80000 usage.