Free Part 36 Calculator: Interest, the 10% and the Relevant Period

How to use the Part 36 calculator

1. Say where the offer was made. Choose a claim, a fixed costs claim (Part 45 Sections VI to VIII) or detailed assessment, and who made the offer.
2. Enter the offer and its service. Enter the amount and the date and method it was sent, ticking 4.30pm or later where that matters. The calculator gives the date the offer was made and the end of the relevant period.
3. Check the relevant period. Leave 21 days, or enter the longer period the offer gave. Add the trial date to check the offer was made at least 21 days before it.
4. Enter what happened. Choose not accepted yet, accepted (with the date and method of the notice of acceptance), or judgment (or the end of the assessment).
5. Enter the result. Enter the damages awarded excluding interest, or the bill as assessed, and the interest awarded, at one rate or as figures.
6. Set the consequences. Where the claimant matched or beat its own offer, choose the enhanced interest rate and, if you like, the costs incurred after the relevant period and the rate of interest on them.
7. Copy the result for the file. Copy the dates, the comparison and each consequence with its rule into the file note.

When the relevant period ends

A Part 36 offer is made when it is served on the offeree (CPR 36.7(2)), so the deemed service rules fix the start (CPR 6.26): an email sent on a business day before 4.30pm is served that day, and one sent later, or on a weekend or bank holiday, on the next business day; first class post and DX are served on the second day after posting if that is a business day, or the next business day after it. Email is good service only where the party served has said in writing that it will accept service by email (PD 6A para 4.1). The relevant period is the period the offer specifies, which must be at least 21 days (CPR 36.5(1)(c)), counted from the day after service; an offer that specifies less is not a Part 36 offer at all (CPR 36.2(2)). An offer made less than 21 days before trial has no fixed period: its relevant period runs to the end of the trial (CPR 36.3(g)(ii)).

Part 36 was rewritten from 6 April 2015. Before then the consequences of judgment were in CPR 36.14 and the costs of acceptance in CPR 36.10, so older decisions, such as Watchorn below, cite the old numbers.

Acceptance and costs

An offer is accepted by serving written notice of acceptance (CPR 36.11(1)), at any time until it is withdrawn (CPR 36.11(2)), though permission is needed while a trial is in progress (CPR 36.11(3)(d)). Accepted within the relevant period, the claimant is entitled to its costs up to the date the notice was served, assessed on the standard basis if not agreed (CPR 36.13(1) and (3)). Accepted afterwards, the court decides costs unless they are agreed, and unless it is unjust the claimant gets its costs up to the end of the relevant period and the party accepting pays the offeror’s costs from then to acceptance (CPR 36.13(4)(b) and (5)). Because the notice is served too, an acceptance emailed after 4.30pm on the last day of the relevant period is late. An offer made less than 21 days before trial is different: on acceptance the court decides the costs unless they are agreed (CPR 36.13(4)(a)).

Did the judgment beat the offer?

CPR 36.17 applies where a claimant fails to obtain a judgment more advantageous than a defendant’s offer, or obtains a judgment at least as advantageous as its own; in a money claim, more advantageous means better in money terms by any amount, however small (CPR 36.17(1) and (2)). A Part 36 offer is treated as including interest up to the end of the relevant period (CPR 36.5(4)), so the comparison takes the sum awarded with the interest accrued by then, and leaves out interest after it (Purrunsing v A’Court & Co [2016] EWHC 1528 (Ch): £470,000 with interest at 3% came to £518,983 at judgment, but to £507,046 with interest only to the end of the relevant period, so it did not beat a £516,000 offer). For an offer made less than 21 days before trial, interest counts up to 21 days after the offer was made (CPR 36.5(4)(b)).

The consequences when a claimant beats its own offer

Unless it considers it unjust, the court must order that the claimant is entitled to (CPR 36.17(4)):

• Enhanced interest on the whole or part of the sum awarded, excluding interest, at up to 10% above base rate for some or all of the period from the end of the relevant period. With interest on the same sum for the same period under any other power, the total rate cannot exceed 10% above base (CPR 36.17(6)), so the calculator shows the ordinary interest for that time and what the two come to together. The rate is not limited to compensation (OMV Petrom SA v Glencore International AG [2017] EWCA Civ 195).
• Indemnity costs from the end of the relevant period, and interest on those costs at up to 10% above base rate, at a rate the court sets separately. How that interest runs is not settled. The Court of Appeal in McPhilemy v Times Newspapers Ltd [2001] EWCA Civ 933 ordered it from the date each item of work was done or each disbursement incurred, as the parties agreed in Bolt Burdon v Tariq; Barry v Essex County Council [2025] EWCC 64 applied it to the total of the costs from the end of the relevant period. The calculator shows the total method, which gives the higher figure.
• An additional amount, once the case has been decided: 10% of the sum awarded up to £500,000 and 5% of anything above, at most £75,000 (CPR 36.17(4)(d)). First-instance decisions differ on whether interest counts as part of the sum: Watchorn v Jupiter Industries Ltd [2014] EWHC 3003 (Ch) left interest out, while Bolt Burdon Solicitors v Tariq [2016] EWHC 1507 (QB) counted contractual interest where no enhanced interest was claimed. So the calculator gives the figure on the damages alone and the figure counting interest up to the end of the relevant period (after that, enhanced interest covers the time). Enhanced interest is not payable on the additional amount (FZO v Adams [2019] EWHC 1286 (QB)), and it applies only to offers made on or after 1 April 2013.

The court looks at each consequence separately, but refusing one needs good reason: beating the offer narrowly, or a substantial cut to a bill, is not enough (JLE v Warrington and Halton Hospitals NHS Foundation Trust [2019] EWHC 1582 (QB)).

When a defendant’s offer is not beaten

Unless it is unjust, the defendant is entitled to its costs from the end of the relevant period, with interest on them (CPR 36.17(3)); the claimant usually keeps its costs up to that date. Where qualified one-way costs shifting applies, the defendant can enforce those costs without permission only up to the damages, interest and costs ordered or agreed in the claimant’s favour (CPR 44.14). None of these consequences applies to an offer that was withdrawn, or changed to be less advantageous where the less advantageous offer was beaten, or that was made less than 21 days before trial unless the court abridged the relevant period (CPR 36.17(7)). The court still takes such offers into account in deciding costs (CPR 44.2(4)(c)).

Part 36 in detailed assessment

Part 36 applies to the costs of detailed assessment proceedings, with the receiving party as claimant, the paying party as defendant, the detailed assessment hearing as the trial and the completion of the assessment as judgment (CPR 47.20(4)), and the proceedings are an independent claim for CPR 36.17 (CPR 47.20(7)). A receiving party that matches or beats its own offer normally gets interest on the bill as assessed at up to 10% above base rate, the costs of the assessment on the indemnity basis with interest, and an extra 10% of the bill as assessed (SCCO Guide 2021, para 16.3; Cashman v Mid Essex Hospital Services NHS Trust [2015] EWHC 1312 (QB)). An offer cannot exclude interest (King v City of London Corporation [2019] EWCA Civ 2266), and unless it says otherwise it is treated as including VAT and the costs of preparing the bill (PD 47 para 19), which the bill itself may claim (PD 47 para 5.19), so the comparison is made on that footing. Interest on costs normally runs at 8% from the date of the costs order (Judgments Act 1838); in the County Court, statutory interest generally applies only to orders for £5,000 or more. Whether the CPR 36.17(6) limit takes in that Judgments Act interest has not been settled, so the calculator shows it separately with what the two come to if it does. To work out the Part 47 timetable itself, use our detailed assessment deadline calculator.

Fixed costs claims

In claims under Part 45 Sections VI, VII and VIII (fixed costs in the fast and intermediate tracks, and noise-induced hearing loss claims), Section II of Part 36 adjusts the costs consequences. Where the claimant beats its own offer, instead of indemnity costs it gets 35% of the difference between the fixed costs for the stage when the relevant period ended and the stage at judgment (CPR 36.24(4) and (5)); the enhanced interest and the additional amount still apply. Where a defendant’s offer is not beaten, the claimant gets the fixed costs for the stage at the end of the relevant period, and the defendant the fixed costs for the stage at judgment, less the claimant’s (CPR 36.24(2) and (9)). Accepted within the relevant period, an offer gives the claimant the fixed costs for the stage at acceptance (CPR 36.23(1)); a defendant’s offer accepted late gives the claimant the fixed costs for the stage when the relevant period ended, and the defendant the fixed costs for the stage at acceptance, less the claimant’s (CPR 36.23(3) and (8)).

Bank of England base rate

Enhanced interest is set by reference to base rate, which the calculator takes as it stood on each day of the period. Base rate is 3.75% (checked 18 October 2026; next decision 5 November 2026). The most recent changes:

Earlier rates back to 1999 are built into the calculator, from the Bank of England’s database.

Related guides

• Late acceptance of a Part 36 offer and the CPR 36.17 uplift
• Attersley: late Part 36 acceptance and the costs regime
• Timokhin: a genuine attempt to settle under Part 36
• Part 36 in housing disrepair costs
• JBX v Frimley: Part 36 and clinical negligence costs
• The terms, defined: Part 36 offer and its costs consequences

Sources

• CPR Part 36: Offers to settle (rules 36.3, 36.5, 36.7, 36.11, 36.13, 36.17 and 36.22 to 36.24)
• CPR Part 47 (rule 47.20) and Practice Direction 47 (paras 5.19 and 19)
• CPR Part 6 (rule 6.26, deemed service) and Practice Direction 6A (para 4.1, service by email)
• CPR Part 44 (rules 44.2 and 44.14)
• The Senior Courts Costs Office Guide 2021 (paras 2.18 and 16.3)
• Bank of England: Bank Rate history
• Purrunsing v A’Court & Co [2016] EWHC 1528 (Ch)
• Watchorn v Jupiter Industries Ltd [2014] EWHC 3003 (Ch)
• Bolt Burdon Solicitors v Tariq [2016] EWHC 1507 (QB)
• McPhilemy v Times Newspapers Ltd [2001] EWCA Civ 933
• FZO v Adams [2019] EWHC 1286 (QB)
• JLE v Warrington and Halton Hospitals NHS Foundation Trust [2019] EWHC 1582 (QB)
• King v City of London Corporation [2019] EWCA Civ 2266
• OMV Petrom SA v Glencore International AG [2017] EWCA Civ 195
• Cashman v Mid Essex Hospital Services NHS Trust [2015] EWHC 1312 (QB)
• Barry v Essex County Council [2025] EWCC 64