How we build and verify every calculator

Our editorial principles

Every calculator on CalcOrchard follows four rules, in order of importance. Break any of these and a tool doesn't ship.

1. Use the right formula. Not an approximation, not something adapted from a similar tool, and not a rough heuristic. The published textbook equation, the HMRC-defined rule, or the IRS-published table.
2. Show your work. Every calculator page includes a plain-English explanation of the formula it uses, an FAQ section covering the edge cases, and (where relevant) a table of the intermediate steps. If you can't reproduce our answer with a pen and paper, we haven't done our job.
3. Respect the user's privacy. Every calculator runs entirely in your browser. Nothing you type is transmitted to our servers, stored in a database, or shared with any third party. We do not require accounts. This is not a nice-to-have; it's a hard requirement.
4. Say what the tool isn't. Every calculator has an explicit list of simplifications and things it doesn't cover. Nobody's edge case is exactly like the defaults — telling you the boundaries of the tool is more useful than pretending it covers everything.

Where the numbers come from

UK tax and National Insurance

All UK tax calculators use the 2024/25 tax year rates and thresholds as published by HMRC. The specific figures we use:

• Personal allowance: £12,570, tapered at £1 for every £2 above £100,000 (fully removed at £125,140).
• Basic-rate band: 20% on income £12,570–£50,270.
• Higher-rate band: 40% on income £50,270–£125,140.
• Additional-rate band: 45% above £125,140.
• Employee National Insurance: 8% on £12,570–£50,270; 2% above.
• Employer National Insurance: 13.8% above the secondary threshold of £9,100.
• Apprenticeship levy (relevant for umbrella): 0.5%.
• Corporation tax: 19% up to £50,000 profit, 25% above £250,000, with marginal relief in between (formula: profit × 0.25 − (250,000 − profit) × 0.015).
• Dividend allowance: £500. Dividend tax: 8.75% basic / 33.75% higher / 39.35% additional.
• Capital gains allowance: £3,000. CGT: 10%/20% for non-property; 18%/24% for residential property.

Scottish rates are used where relevant on the Scottish take-home page. If you're on a K-code, marriage allowance transfer, or any other non-standard tax code, the calculator will state that as a limitation up front.

US tax and retirement

US calculators use the 2024 tax-year figures published by the IRS: single-filer standard deduction $14,600, married filing jointly $29,200; brackets 10/12/22/24/32/35/37%; IRA/Roth contribution limit $7,000 (under 50) / $8,000 (50+); 401(k) employee limit $23,000 / $30,500. Where a projection extends past 2024, we hold rates constant unless we state otherwise.

Finance formulas

Compound interest, present value, future value, loan amortisation, and other standard finance formulas come from the textbooks — for example, compound interest with regular contributions uses the standard closed-form expression FV = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) − 1) / (r/n)]. No shortcuts, no approximations that only work in a particular range.

Health calculators

The BMR calculator uses the Mifflin-St Jeor equation (1990), currently considered the most accurate BMR predictor for the general adult population. The body fat calculator uses the US Navy circumference method. BMI uses the WHO adult classification (underweight < 18.5, healthy 18.5–24.9, overweight 25–29.9, obese ≥ 30). Every health tool is explicitly framed as a rough population-level estimate, not a medical assessment.

How each calculator gets verified

Before a tool ships, we run it through the following checks:

• Test with at least two independent published worked examples (HMRC, IRS, or textbook).
• Test the edge cases — zero input, very small and very large numbers, negative where relevant, and the exact boundaries of tax bands.
• Sanity-check against at least one other trusted calculator (for example, our UK take-home calculator against listentotaxman.com, our loan payment calculator against the standard amortisation schedule).
• Read the final rendered page as if we'd never seen it before — is the answer understandable? Is the explanation of the formula clear?

When rates change

UK tax rates are refreshed after each Budget and Autumn Statement. US tax rates are refreshed after each annual IRS Revenue Procedure publication (typically October–November each year). We date-stamp the "rates as of" note on each calculator page. If we haven't updated a calculator that should have been updated, please let us know — we take corrections seriously and will fix quickly.

What we won't do

• Require you to create an account to use a calculator.
• Send your inputs to our server or any third party.
• Publish a calculator without stating the formula and its limits.
• Pretend a tool is personalised financial, tax, or medical advice. It isn't. It's a well-built estimation aid.
• Silently change how a calculator works. Meaningful changes are noted in the tool's FAQ or explainer.

Reporting a problem

If you spot a number that looks wrong, a formula that doesn't match the source, an unclear explanation, or a broken page, please tell us via the contact form. Reader-reported corrections are how the site improves. Anything material is fixed within a few days.

Privacy in one line

We do not store your calculator inputs. Full details in the privacy policy.

Content on CalcOrchard is for general information and education only. It is not tax, medical, legal, or personalised financial advice. Regulations change. Individual circumstances matter. Always consult a suitably qualified professional before acting on anything you learn here.