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Measured on six things

Free UK calculator for statutory late payment interest an...

Security

HTTPS enforced with SPF and DMARC, but zero security headers beyond that — no HSTS, CSP, frame protection, or referrer policy. Header checklist 4/12. No published audits.

Screenshots

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UK Late Payment Interest Calculator

About this app

The UK Late Payment Interest Calculator is a free browser-based tool designed to help freelancers, contractors, and small business owners enforce their rights under the Late Payment of Commercial Debts (Interest) Act 1998. It calculates statutory interest at the current 11.75% rate and applies fixed compensation of £40, £70, or £100 based on invoice value. The tool automatically determines the correct due date using agreed terms or the 30-day default, then computes simple daily interest from the day after the due date. The standout feature is its built-in chasing email generator, which drafts a professional, ready-to-send message pre-filled with your figures, saving time and reducing awkwardness. A rate history tab shows reference rates back to 2019 for older debts, while an override option handles pre-2019 cases. The calculator also educates users on legal nuances, such as when to charge a public body, how to claim per invoice, and why interest is outside the scope of VAT. I found ...

Key features

1Statutory interest calculator at the current 11.75% rate (Bank Rate 3.75% + 8%) with automatic due-date determination from agreed terms or the 30-day default.
2Built-in chasing email generator that pre-fills your figures, business name, customer name, and invoice number into a ready-to-send draft.
3Rate history tab covering half-yearly statutory rates back to 2019, plus an advanced override for pre-2019 debts.

Target customer profile

Drawn from the product itself, not from a survey.

Demographic

Freelancers and sole traders in creative industries

Pain points

Reluctant to chase clients for fear of damaging relationships; lack of legal knowledge about statutory rights; time consumed by manual interest calculations.

Primary needs

Quick, accurate interest and compensation figures; professional chasing email templates; confidence to assert legal rights without conflict.

Demographic

Small business owners and finance managers in the UK

Pain points

Persistent late payments straining cash flow; inconsistent invoicing and follow-up processes; difficulty tracking multiple overdue invoices.

Primary needs

Standardised tool for calculating claims per invoice; clear documentation for escalation; easy integration into existing finance workflows; up-to-date statutory rates.

Demographic

Accounts receivable and credit control staff in SMEs

Pain points

Manual calculation errors; missed deadlines for escalating claims; lack of ready-made templates for varied scenarios (no terms, public sector customers).

Primary needs

Automated due date and rate determination; rate history lookup; generation of customised emails with accurate figures; legal guidance summary for compliance.

Scout Review

Written by AI from measured evidence, scored out of 100.

The UK Late Payment Interest Calculator is a genuinely useful free tool that solves a real, annoying problem for UK freelancers and small businesses: working out what you're owed under the 1998 Act and then actually asking for it. The three-step flow is clean, the defaults are sensible, and the chasing email generator is the standout feature — it turns a passive-aggressive draft into a one-click copy. The rate history back to 2019 and the pre-2019 override show real care for edge cases. Where it falls short: it's a sub-page of a Bristol marketing agency's site with no product identity of its own, mobile performance is dragged down to 69/100 by the parent site's scripts, and the security headers are 4/12 with no published audits. Accessibility is 94/100 with three fixable failures. Growth is capped by the fact that it's a lead-gen tool, not a standalone product — but as a lead-gen tool, it's a smart one. If you're a UK freelancer with an overdue invoice, this saves you twenty minutes and a difficult conversation. That's worth more than most paid tools deliver.

Measure by measure

Usability

Three-step flow with sensible defaults, a one-click copy for the chasing email, rate history to 2019, and a pre-2019 override. No signup, no paywall, instant value.

Design

Clean three-tab calculator with numbered steps and clear labels, but it lives inside a Bristol agency's site with shared nav and no distinct product identity. Functional, not memorable.

Speed

Desktop Lighthouse 98/100 but mobile only 69/100 with LCP 3.2s and TBT 456ms — the parent site's scripts drag the tool down on phones, where freelancers actually check invoices.

Security

HTTPS enforced with SPF and DMARC, but zero security headers beyond that — no HSTS, CSP, frame protection, or referrer policy. Header checklist 4/12. No published audits.

Accessibility

Lighthouse a11y 94/100, but three real failures: prohibited ARIA attributes, non-sequential headings, and no main landmark. No accessibility statement or WCAG claim published.

Growth

Real, persistent UK problem with a clear wedge: the integrated chasing email and rate history that GOV.UK and Simply Business don't offer. Niche but genuinely underserved.

Improvement areas

Security headers are 4/12 — no HSTS, CSP, frame protection, or referrer policy despite handling invoice figures and generating emails.
Mobile Lighthouse performance is 69/100 (LCP 3.2s, TBT 456ms) because the parent agency site's scripts load on the calculator page.
Accessibility has three fixable failures: prohibited ARIA attributes, non-sequential headings, and a missing main landmark.

Design & Usability

The tool is a clean, three-tab calculator bolted onto a Bristol marketing agency's website, sharing its nav with 'Recruitment' and 'Education' links. That's the design story: functional, legible, and completely lacking a product identity of its own. Numbered steps (Invoice → Payment terms → Payment status) guide you through without confusion, and the 'No terms agreed' default correctly falls back to the 30-day statutory rule. Usability is where it earns its keep. No signup, no paywall, no email gate — you land, you type an invoice amount, you get a figure. The chasing email generator pre-fills your business name, customer name, invoice number, and the calculated interest, then hands you a one-click copy button. For a freelancer who has been avoiding a difficult conversation for three weeks, that's the entire value proposition in one feature. The rate history tab going back to 2019, plus an advanced override for pre-2019 debts, shows the maker actually thought about edge cases rather than shipping a toy. The educational copy explaining the reference-date rule and per-invoice compensation is a genuine bonus. Where it stumbles: the parent site's chrome bleeds in, there's no way to save or export a calculation, and the email template is a single fixed draft rather than a few tone options. Still, for a free tool, this is more thought-through than most paid ones.

Speed & Security

Speed is a tale of two devices. Lighthouse desktop scores 98/100 — the calculator itself is light. Mobile scores 69/100 with LCP 3.2s, FCP 2.9s, and TBT 456ms, which points at the parent agency site's scripts and fonts dragging the page down on phones. Since freelancers chasing invoices are exactly the people checking this on a phone between jobs, that mobile number matters more than the desktop one. The fix is boring and effective: defer non-critical JS, subset the fonts, and the tool would likely clear 85 on mobile. Security is the weakest dimension. HTTPS is enforced, and SPF and DMARC records exist, which is the bare minimum for a site collecting business names and invoice figures. But the HTTP probe found no HSTS, no Content-Security-Policy, no X-Frame-Options, no X-Content-Type-Options, no Referrer-Policy, and no Permissions-Policy — 4 out of 12 checklist points. For a tool that handles financial figures and generates emails, that's thin. There's no published SOC 2, ISO 27001, or pen-test report, and no security page. None of that is unusual for a free tool from a small agency, but it does mean the security posture is 'it works' rather than 'it's been checked'. Adding HSTS and a basic CSP would take an afternoon and move this from weak to respectable.

Accessibility & Growth

Accessibility lands at a solid-but-imperfect 94/100 on Lighthouse, with three real failures worth naming: prohibited ARIA attributes, headings that don't descend sequentially, and a missing main landmark. Those are the kind of issues that don't show up visually but do break screen reader navigation — a blind user tabbing into the page has no landmark to jump to and has to wade through the parent site's nav to reach the calculator. There's no accessibility statement and no WCAG conformance claim, and the tool is English-only. None of that is fatal for a free UK-focused utility, but the heading order and main landmark are genuinely five-minute fixes. Growth is the interesting one. The problem is real and permanent: UK B2B late payments are a chronic cash-flow wound for freelancers and small businesses, and the 1998 Act gives them a statutory right most don't know how to exercise. The alternatives — GOV.UK's calculator, Simply Business's, Farillio's — all do the maths. None of them draft the email. That's the wedge: this tool doesn't just tell you what you're owed, it writes the awkward message for you, which is the actual barrier to claiming. The rate history back to 2019 and the pre-2019 override add credibility for older debts. The limits are real: it's a free lead-gen tool for a Bristol agency, not a standalone product, so there's no obvious monetization path and no network effect. It can grow as a traffic magnet and a credibility play for the parent consultancy, but it won't become a business on its own. That caps the ceiling without making the idea weak.

Conclusion

The honest verdict: this is a well-built free utility that punches above its weight on the one feature that matters — the email generator — and below its weight on the plumbing. If the maker spent a weekend adding HSTS, a CSP, a main landmark, and sequential headings, the scores would jump across three dimensions at once. The bigger question is whether it ever becomes more than a lead magnet for the parent agency. As it stands, it's the best free UK late-payment calculator with an email generator that I could find, and that's a specific enough niche to be worth something. Use it, copy the email, get paid. Just don't expect it to remember your last calculation, because it won't.

Suggestions for improvement

Add HSTS, Content-Security-Policy, X-Frame-Options, and Referrer-Policy headers — an afternoon's work that lifts security from 4/12 to near-full marks.
Fix the three Lighthouse a11y failures: remove prohibited ARIA attributes, correct heading order, and add a <main> landmark.
Defer non-critical JS and subset fonts on the parent site to push mobile Lighthouse from 69 toward 85+.
Add 2-3 email tone variants (firm, friendly, final notice) and a save/export option for the calculation.

How we score

Alternatives comparison

Named competitors, point by point. Nobody paid to appear here or to be left out.

	UK Late Payment Interest Calculator	GOV.UK Late Payment Interest Calculator	Simply Business Late Payment Calculator	Farillio Late Payment Interest Calculator
Calculates statutory interest under the Late Payment of Commercial Debts (Interest) Act 1998	Yes — 11.75% current rate, automatic due-date logic, simple daily interest	Yes — official GOV.UK calculator	Yes — basic interest calculator	Yes — calculator bundled with legal-tech platform
Generates a ready-to-send chasing email pre-filled with your figures	Yes — built-in email generator with one-click copy	No — guidance only, no email drafting	No — calculator only	Partial — templates available but often behind subscription
Historical statutory rate lookup for older debts	Yes — half-yearly rates back to 2019 plus pre-2019 override	Partial — guidance references rates but no interactive history tab	Limited — fewer educational resources	Partial — depends on subscription tier
Free to use with no account or paywall	Yes — fully free, no signup	Yes — free government tool	Yes — free insurer-provided tool	No — full features often behind subscription
Educational guidance on the 1998 Act and legal nuances	Yes — explains reference dates, per-invoice compensation, VAT treatment	Yes — official statutory guidance	Limited — fewer educational resources	Yes — legal-tech platform with contract templates