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Flat rate vs monthly plan calculator

A monthly fee buys you a lower rate. This shows whether that trade is worth it at your takings, and the exact point where it starts paying for itself.

£
£
Flat rate
%
p
Monthly plan (or interchange++ average)
£
%
p

How the break-even works

Every pound you take costs a little on each option: the percentage, plus the fixed fee spread over your average sale. Call that the marginal rate. The plan charges a monthly fee but has a lower marginal rate, so it saves a little on every pound and needs enough pounds to cover its fee.

Break-even takings = monthly fee ÷ (flat marginal rate − plan marginal rate), where marginal rate = percentage + fixed fee ÷ average sale. Above that level of takings the plan is cheaper; below it, the flat rate wins. If the plan's marginal rate isn't lower, it never pays off.

Fixed per-sale fees matter more than they look. A 5p fee on a £25 average sale adds 0.2 percentage points to the plan's real rate; on a £5 sale it adds a full percentage point, which can wipe out the plan's advantage altogether.

A note on interchange++

Interchange is set by the card issuer and differs by card type (consumer debit, consumer credit, commercial, overseas). Interchange++ passes those through at cost plus the scheme fee plus the provider's margin. For UK consumer cards, interchange is capped by regulation, so the variable part is mostly scheme fees and the provider's margin. We don't quote specific cap levels here until we've verified the current figures from the Payment Systems Regulator. In practice, use your statement's average all-in rate.

Worked example

With illustrative figures of 1.69% flat against £20 a month + 0.99% + 5p, at a £25 average sale: the plan's marginal rate is 0.99% + 5p ÷ £25 = 1.19%. Break-even = £20 ÷ (1.69% − 1.19%) = £4,000 a month. At £5,000 the flat rate costs £84.50 and the plan £79.50; at £2,500 the flat rate costs £42.25 and the plan £49.75.

Frequently asked questions

What is a flat rate?
One percentage on every card payment (sometimes plus a small fixed fee), whatever the card type. It is simple and predictable, and usually comes with no monthly fee.
What is interchange++ (interchange plus plus)?
A pricing model where you pay the actual interchange fee set by the card issuer, plus the card scheme fee (Visa or Mastercard), plus the provider's own margin. Because interchange varies by card type, your rate changes from sale to sale. It can be cheaper for higher-volume businesses but is harder to predict.
Can I use this calculator for interchange++ pricing?
Yes, approximately. Enter the average all-in rate from a recent interchange++ statement (total fees ÷ total takings, minus any fixed fees) as the plan rate. The result is only as good as that average.
What if the break-even is above my takings?
Then the flat rate is cheaper for you today. Re-run it if your takings grow, or check the fees calculator to compare named providers.

Last reviewed 14 October 2026 by Oliver Burton. Editorial policy.