The specific moment I decided to build this was when I was trying to work out whether a contract offer was worth taking. I wanted to compare take-home on an outside IR35 Ltd company contract versus the same rate inside IR35 through an umbrella. Simple question. Should take five minutes.
Forty minutes later I had visited six websites. Two wanted an email address before showing results. One had clearly year-old rates. One gave wildly different numbers with no explanation of how it got there. One crashed on mobile. I'm a solutions architect who works with financial services clients. I know what a reasonable digital tool looks like — and none of those were it. So I built what I actually wanted.
What was wrong with the existing tools
Most of the "calculators" I landed on were lead generation forms wearing a calculator costume. The calculation is a pretext for capturing a name, an email, a phone number, and preferably a day rate so someone can call you about an accountancy package. That might be a fine business. It is a terrible way to answer a five-minute question.
A lot of them were out of date, and they did not tell you. The employer NI threshold dropped in April 2025. The mileage rate went up in May 2026. Months later, sites were still running the old numbers with no last-updated date anywhere on the page. A take-home figure built on last year's thresholds is worse than no figure at all, because it looks precise.
The ones that did produce a number often produced only the number. A single take-home figure with no working is almost useless for a decision. You cannot tell whether they treated employer NI as a company cost, whether they used the £500 dividend allowance or the old £2,000 one, whether they assumed a salary at the NI threshold or at the personal allowance. If you cannot see the working, you cannot trust the bottom line.
And a surprising number were not actually free. The headline number was free. The detail was paywalled, or you needed the app, or you had to create an account that existed solely so they could collect data. I wanted to type some numbers and leave. That should not require a product relationship.
What I decided to build instead
No email capture, no account creation, no paywalls. You arrive, you enter numbers, you get an answer, you leave. If the tools are useful you will come back. If they are not, capturing your email would not have made them useful.
Show the working. Every calculator should produce a full tax breakdown, not just a take-home figure. If the corporation tax line looks wrong, you can see it. If the dividend tax looks high, you can see which band it landed in. A bottom line without a breakdown is a magic trick.
Stay current. All rates live in a single config file. When HMRC changes a threshold, one update propagates through every calculator at once. That is not a technical flex. It is how you avoid the "this site still thinks the mileage rate is 45p" problem that made me start.
Handle the edge cases that the other tools skipped. The IR35 calculator shows all three scenarios — outside Ltd, inside through the company, umbrella/PAYE — on the same page. The mileage calculator handles the 10,000-mile split. The Ltd versus sole trader calculator shows the actual crossover for your expense level, not a generic £30,000 rule of thumb that stopped being true years ago.
What I learned building it
The IR35 surprise: at certain income levels, around £500 a day, employee PAYE inside IR35 can actually beat operating through a Ltd outside IR35. Employer National Insurance reduces the taxable base before income tax is calculated, and that mechanical effect is large enough, in a specific band, to outweigh the dividend advantage. It is under-discussed, partly because it feels backwards, and partly because most calculators only show you the scenario you already assumed you were in.
The mileage rate is an embarrassment. 45p was set in 2011. Inflation-adjusted, it should have been somewhere around 65–70p by 2026. The increase to 55p is welcome. It is also catching up, not leading. Contractors who drive for work have been under-compensated in real terms for over a decade, and the new rate does not close the gap.
The dividend allowance impact is bigger than the coverage suggested. The old "Ltd from £30,000" crossover is no longer accurate. A typical crossover now sits around £50,000–£60,000 once you count a realistic accountant. A lot of people incorporated on advice that was right in 2018 and is wrong in 2026. Building the Ltd versus sole trader calculator is what made that concrete for me — not a think piece, a spreadsheet that kept refusing to match the folklore.
The stack, for anyone who's interested
Plain HTML, CSS and JavaScript. No framework. No build step. No database. All calculations run in the browser. Hosted on Cloudflare Pages. That was a deliberate choice: no dependencies to update, no npm vulnerabilities to patch, no serverless timeouts to debug. For a tool site where the core value is a calculation rather than a UI framework, this is the right call. It loads fast, it works offline once cached, and there are no JavaScript frameworks weighing more than the rest of the site.
What's live and what's coming
Nine calculators are live. The VAT calculator adds or removes VAT, including the flat rate scheme. The mileage calculator applies HMRC approved rates with the 10,000-mile split. The invoice generator creates a PDF with no signup. The day rate calculator converts between day rate and salary equivalent. The IR35 calculator compares Ltd, umbrella and PAYE take-home. Ltd versus sole trader shows which structure is more tax efficient at your numbers. The self-assessment estimator estimates your bill before you file. The dividend calculator finds an optimal salary and dividend split. The contractor tax year planner lays out month-by-month set-aside and payment deadlines.
There are also 15 guides covering the same ground in prose — Self Assessment, IR35, expenses, Making Tax Digital, corporation tax, and the rest. All of it is free, and it will stay free.
Feedback welcome
If you spot a calculation that looks wrong, a rate that's out of date, or a scenario the tools don't handle well — I genuinely want to know. The point of the site is to be useful and accurate, and both require real-world feedback. Email [email protected].