Web tools that do one job properly.
16 free tools across finance, technology, developer, image and everyday categories — no account, no upload for anything that can run locally, and every calculator shows the formula behind its result instead of a black-box answer.
Featured tools
A starting point across categories — each built out in full with worked examples, stated assumptions and an explanation of its method.
- Mortgage CalculatorEstimate the monthly payment and total interest on a repayment mortgage.
- Compound Interest CalculatorProject how savings grow when interest compounds and you contribute regularly.
- Business Loan CalculatorCalculate business loan repayments and the real cost once fees are included.
- Profit Margin CalculatorCalculate gross margin, markup, and the price needed to hit a target margin.
- SaaS ROI CalculatorEstimate the payback period and net return on a SaaS subscription.
- Cloud Cost CalculatorEstimate monthly cloud spend across compute, storage and data transfer.
- Software TCO CalculatorCompare the full multi-year cost of two software options side by side.
- Password GeneratorGenerate a strong random password locally, without sending anything to a server.
Guides
All 10 guidesSourced explanations of the formulas and standards behind the tools above — how mortgage amortization actually works, margin vs markup, QR error correction, and more.
- Margin vs Markup: What They Actually MeasureTwo numbers, the same profit, and one very common pricing mistake.
- How Mortgage Amortization WorksThe same payment every month, but a completely different split between interest and capital.
- How Loan Fees Change the Real Cost of BorrowingThe advertised rate and the real cost of a loan are rarely the same number once fees are counted.
- Compound Interest, Explained With NumbersThe slow, unglamorous first decade is what makes the second one work.
- How to Calculate the Real Cost of a SaaS SubscriptionThe number on the invoice and the number that determines whether the software was worth buying are usually different.
Finance
All finance toolsCalculators for borrowing, saving and running a business, each showing the formula it uses and the assumptions behind the result.
- Mortgage CalculatorEstimate the monthly payment and total interest on a repayment mortgage.
- Compound Interest CalculatorProject how savings grow when interest compounds and you contribute regularly.
- Business Loan CalculatorCalculate business loan repayments and the real cost once fees are included.
- Profit Margin CalculatorCalculate gross margin, markup, and the price needed to hit a target margin.
Technology
All technology toolsCost and return models for software and infrastructure decisions, built for people who have to justify the spend.
Developer
All developer toolsEveryday encoding, formatting and generation tools that run entirely in your browser.
Image & PDF
All image & pdf toolsFile tools that compress, resize and convert without uploading your files to a server.
Everyday
All everyday toolsConversions, counts and quick calculations — the things people look up several times a week.
- Percentage CalculatorCalculate percentages, percentage change, and one number as a percentage of another.
- Word CounterCount words, characters, sentences and paragraphs in text as you type.
- Age CalculatorCalculate exact age or the interval between two dates in years, months and days.
- Unit ConverterConvert between metric and imperial units of length, weight, volume, area and temperature.
Why these tools
Every calculator on this site states the formula it uses and what it holds constant, rather than showing a number with no working behind it. Where a task can run entirely in your browser — the image, developer and everyday tools — it does, which means nothing you enter or upload is sent anywhere. See how tools are built and checked and what UseKitl is for the detail behind both of those claims.