Bounded interest estimate
Estimate one fixed-balance, fixed-rate Corporation Tax interest interval—not the exact HMRC account charge.
HMRC's published clerical /366 basis; ordinary late-payment interest only. Rate history and payment allocations are not fetched. Sources reviewed 28 September 2026.
Estimate ordinary Corporation Tax interest for one fixed balance, one manually entered annual rate and one verified interval. Changing rates, part-payments and HMRC account adjustments need separate treatment.
It applies simple daily interest to the entered tax balance, annual rate and chargeable days. It explains all three, uses HMRC's published /366 clerical basis and rounds the final interval amount to pennies.
It does not calculate the complete interest on an account whose balance or rate changes. If either changes, select split interval and calculate each verified interval separately. No / unsure on the scope question withholds a monetary result.
Rule summary
HMRC COM80010 makes the normal CT due date non-interest-bearing: the ordinary period starts the next day and runs through effective payment. Payment on the due date gives zero days; one day later gives one. Calendar days are counted in UTC; leap day counts as a day.
DMBM404010 publishes amount × annual rate × days / 366 for clerical calculation. It describes due date included/payment date excluded; for a constant rate and balance this has the same elapsed day count as the CT endpoint description. Do not use that equivalence to allocate changing-rate days: use the actual rate on its change date.
For a split interval, enter the day before its first chargeable day as the start boundary and the last chargeable day as the end. A new rate beginning 1 November therefore starts a new interval with boundary 31 October. Neither gaps nor overlapping chargeable days should be introduced.
Rates change: check the official HMRC schedule for each interval, not just today's rate. Part-payments change principal: verify HMRC's effective dates and allocations, then split balance intervals. The tool neither retrieves rates nor models a payment ledger.
Interest is not a late-filing penalty and is not compounded here. The payment due date is separate from the CT600 return-filing deadline. Quarterly-instalment debit/credit interest and repayment interest are different regimes outside this estimate.
Worked examples
Illustrative fixed annual rate 8%, tax £10,000, due 1 October 2026, effective payment 30 November 2026. Assume no intervening rate or balance change.
2 October through 30 November inclusive gives 60 days. £10,000 × 0.08 × 60 / 366 = £131.147540…; final interval estimate £131.15. The rate is illustrative, not a claim about HMRC's actual rate on these dates.
£10,000 at an illustrative fixed 8%, due 1 October 2026. Compare payment on 1 October with payment on 2 October.
Payment on the due date: 0 days, £0.00. Payment one day later: 1 day, £10,000 × 0.08 / 366 = £2.185792… → £2.19. The due date is not a chargeable late day.
Illustrative rates only: £10,000 due 1 October, paid 30 November 2026; 8% until 31 October and 9% from 1 November. The new rate applies on the change date.
Split mode: boundary 1 October/end 31 October gives 30 days at 8% = £65.57. Boundary 31 October/end 30 November gives 30 days at 9% = £73.77. Sum of rounded interval estimates: £139.34. Do not apply one current rate to all 60 days; an unrounded aggregate can differ by pennies.
Illustrative fixed 8%: £10,000 due 1 October, £4,000 paid effectively on 31 October, remaining £6,000 paid on 30 November 2026. Assume HMRC's allocation charges the initial balance through that effective payment day.
Verify the payment allocation first. Boundary 1 October/end 31 October: £10,000 for 30 days = £65.57. Boundary 31 October/end 30 November: £6,000 for 30 days = £39.34. Rounded interval sum £104.91. A single £10,000 balance for 60 days would overstate this example; this tool does not simulate the account.
Verify the actual due date in the separate CT payment-deadline tool and the effective payment date on HMRC records.
Check each rate against the official HMRC schedule; split at rate and balance changes using the guide examples.
Reconcile the estimate to HMRC's computation. A disputed interest charge has an enquiry/objection route, not the ordinary statutory penalty-appeal route.
Read the guide
Read the supported rule, assumptions and worked examples behind this tool.
Assumptions and limits
This tool gives a practical estimate and depends on the assumptions shown below.
Reviewed on . Preserved source review from the accepted scoped statutory repair; this contract migration does not re-date that review.
Applies to: Ordinary CT late-payment interest; HMRC clerical /366 basis. User must supply the applicable historical annual rate.
Reviewed scope: One fixed principal, one annual rate and one verified interval; no rate history, part-payment ledger, instalment regime or exact HMRC account balance.
Reviewed model: corporation-tax-interest-calculator: Ordinary CT late-payment interest; TMA 1970 s87A; fixed-rate clerical /366 estimate; sources reviewed 28 September 2026
No. You must enter the rate applicable to the whole interval. If rates change, use split intervals and the official rate schedule.
No. It is a transparent fixed-balance, fixed-rate interval estimate. Payment allocations, account adjustments, different regimes and HMRC rounding are not simulated.
The estimate follows DMBM404010's published clerical /366 formula, not a calendar-year denominator selected by JavaScript. HMRC's illustrative CT207 LP row (£75,000, 6.8%, 1,187 days, £16,540.16) corroborates that arithmetic basis. Its printed dates are inconsistent, so it is not used as a date-count example or actual historical rate evidence.