How SparkCalc Works

Where our numbers come from

Every calculator is built on a documented formula or an established public dataset, not on guesswork. We cite the primary source directly on each calculator page under How We Calculate This and in a References list, so you can check the methodology yourself. Examples of the sources we rely on:

• Health & fitness: the World Health Organization, the U.S. Centers for Disease Control and Prevention, the National Institutes of Health, and peer-reviewed equations such as Mifflin–St Jeor.
• Family & pregnancy: WHO growth standards, the American College of Obstetricians and Gynecologists, and U.S. Department of Agriculture data.
• Money & finance: the U.S. Securities and Exchange Commission (Investor.gov), the Consumer Financial Protection Bureau, the Bureau of Labor Statistics, and the IRS.

Calculations run in your browser

Every calculation happens entirely on your device. We do not collect, store, or transmit the values you enter. Nothing you type into a calculator ever leaves your browser, which is why you can use SparkCalc without an account and without sharing personal data. See our Privacy Policy for details.

How calculators are built and checked

Each calculator is defined by its formula in code and is covered by an automated test suite that checks its outputs against known, worked examples. When we change a formula or add a calculator, those tests must pass before it goes live. This helps catch arithmetic and edge-case errors before they reach you.

How often we review

We review our calculators’ methodology and source data periodically, and whenever an underlying standard, rate, or dataset is updated. Our higher-stakes calculators — those covering health, money, and family decisions — show a “Methodology last reviewed” date so you can see how current the information is.

Accuracy and limitations

We work hard to be accurate, but our calculators are for general education and information only. They produce estimates based on the inputs you provide and the assumptions built into each formula, and they cannot account for your full personal circumstances. They are not a substitute for advice from a qualified professional — a doctor, financial adviser, or other expert — especially for important health or money decisions.

Spotted a problem?

If you think a calculator is wrong, or you can point us to a better source, we want to hear about it. Please get in touch — corrections from readers genuinely help.