Stable Numbers

Stable Numbers strips away everything that gets between you and the answer. Each calculator loads in under half a second, works offline once cached, and delivers results as you type. There's no account to create, no email to verify, and no premium tier dangling better features behind a paywall. Every tool on the site is free and will stay that way.

The difference starts with the interface. Instead of cramming multiple functions into one mega-calculator, each tool does one thing perfectly. The wallpaper calculator tells you how many rolls to buy. The loan calculator shows your monthly payment. The percentage increase calculator works out the rise between two numbers. Clear labels, sensible defaults, and instant feedback mean you spend seconds getting answers, not minutes figuring out which button to press.

Behind the scenes, every formula has been checked against official sources and tested with edge cases that trip up other calculators. When you're working out VAT, the tool uses HMRC rates. When you're converting units, it uses the exact definitions from standards bodies. And when there's ambiguity - like whether a month means 30 days or 4 weeks - the calculator shows both interpretations so you can pick the right one for your situation.

Every calculator on Stable Numbers uses precise mathematical formulas, not approximations or rules of thumb. Take the compound interest calculator: it uses the exact formula taught in financial mathematics, accounting for daily, monthly, or annual compounding periods down to the penny. The date calculators handle leap years, varying month lengths, and timezone transitions. The conversion tools use definitions from the International System of Units, not rounded values that accumulate errors.

Where real-world complexity exists, the tools show it rather than hiding it. The income tax calculator breaks down exactly which portions of your salary fall into which bands. The mortgage calculator separates principal from interest in every payment. The percentage calculators show the steps, so if you're checking someone else's figures, you can see exactly where any differences creep in.

Regular testing keeps everything accurate. Automated checks run against known values - like verifying that the loan calculator matches bank statements, or that the hex converter produces the same results as programming languages. When tax rates change or new regulations come in, the relevant tools update within days. If you spot something that looks wrong, there's a feedback link on every page that goes straight to a human who will investigate within 24 hours.

The percentage calculator suite covers the most common daily calculations. Need to work out a tip? The percentage calculator does it in one click. Comparing prices after a discount? The percentage decrease calculator shows the saving and the final price. Working out markup for your business? There's a specific tool that converts between margin and markup, showing the formula so you can explain it to others.

For money matters, start with the loan repayment calculator if you're comparing finance options. It shows total interest paid, not just monthly payments, which reveals the true cost of extending the term. The compound interest calculator helps with both savings and debt - use it to see why overpaying your mortgage by £50 a month could save thousands, or how much your ISA might grow by retirement.

The conversion tools handle the measurements that trip everyone up. Metric to imperial for recipes and DIY. Celsius to Fahrenheit when checking foreign weather forecasts. Currency conversion with live rates for travel planning. Each converter shows the exact multiplier being used, so you can do quick mental approximations once you've seen the precise figure a few times.

Every tool is free for commercial use, whether you're a sole trader checking VAT calculations or an accountancy firm verifying client figures. There's no license to buy, no attribution required, and no limit on how many calculations you can run. The tools work identically whether one person or a thousand people are using them, with no throttling or queues during busy periods.

Several calculators are specifically built for business scenarios. The margin calculator helps with pricing decisions, showing how different markups affect your bottom line. The invoice calculator handles VAT, discounts, and multiple line items. The loan calculators work for both personal and business borrowing, with options for balloon payments and variable rates. The percentage tools help with everything from calculating commission to working out year-on-year growth.

For teams that need branded calculators on their own sites, Stable Numbers offers custom development at a straightforward hourly rate. These can match your visual identity, integrate with your systems, and include specific formulas for your industry. Past projects include BTL yield calculators for estate agents, recipe scalers for food manufacturers, and pricing tools for subscription services. Each one delivered as clean, maintainable code you own outright.

The request form at /submit-a-tool/ goes straight to the developer, not a support queue or chatbot. Describe what you need to calculate and why existing tools don't quite fit. The more specific the better - "a calculator for working out fabric needed for roman blinds" is more helpful than "a sewing calculator". Include any formulas you're currently using, examples with real numbers, and links to similar tools that get close but not quite right.

Popular requests get built first, but niche tools have their place too. Recent additions from user suggestions include a soil volume calculator for raised beds, a points calculator for UK driving licenses, and a tool for working out how many bathroom tiles to order with wastage. Each one serves hundreds of people who had the same problem but hadn't asked. Your specific need probably matches someone else's.

Updates to existing tools are just as welcome as new requests. If a calculator needs another input field, a different unit option, or clearer labels, mention it. Many improvements come from users pointing out edge cases - like the date calculator now handling business days after someone needed to exclude weekends, or the percentage calculator showing decimal places after teachers requested more precision for marking. Small changes that make tools more useful get implemented quickly, often within a week of the suggestion.