What started as a personal tool
I made the BMI calculator first because doctors kept telling me I was overweight at 178 cm and 82 kg. The number rolls in at 25.9, which is the official 'overweight' bracket. The same BMI belongs to a roommate I once had who could barely walk a flight of stairs without stopping. We had different bodies. The metric did not care. I built the calculator with a visible note that BMI was designed in the 1830s for population statistics, not individual health, and that single sentence has gotten more thank-you emails than the rest of the site combined.
From there it grew the way personal tools always grow: I built the next one when I needed it twice in the same week. The percentage calculator came after a tax season where I kept misreading 'percentage points' versus 'percent change.' The currency converter came on a trip where I bought groceries three times and could not feel whether I was overpaying. The presentation timer came the day a friend asked me to time her keynote rehearsal because she could not see her phone from the lectern.
None of these tools are clever. They are all things you could do in a spreadsheet in five minutes. The point is that you should not have to. You should be able to type a number and get the right answer back in the right unit, on the device you already have open.
Why each calculator exists
The age calculator started as a wedding-RSVP problem: I needed to know exactly how many days my friend's grandmother had been alive to put the number on a card. The answer was 31,287, in case you are curious. The anniversary calculator was built the next week because that same calculation kept being useful for entirely different reasons.
The BMI calculator deliberately shows a body-fat-percentage estimate alongside it (US Navy method, with both metric and imperial inputs), because BMI alone misled me for years. The calorie calculator implements the Mifflin-St Jeor equation, which is the one most clinical dietitians use as of 2026, and shows you the value next to the older Harris-Benedict number so you can see where the 6-9% gap goes.
The loan calculator and tax calculator are the most opinionated. The loan tool shows you the total interest paid as a fraction of the loan, in big text, because that is the number my own brain refused to compute when I bought my first car. The tax calculator separates marginal rate from effective rate prominently, because conflating those two costs people thousands of dollars every year.
What we do not track
No analytics that knows who you are. We use Google Analytics 4 with IP anonymization on, advertising features off, and Google Signals off. There is no behavior profile attached to your visit, no advertising ID, no cross-site tracking. The only thing the analytics records is that someone in roughly your country opened roughly which calculator and roughly when. That is enough for us to know which tools to keep maintaining.
Calculations never leave your browser. The math runs in JavaScript on your device. We do not have a server-side calculation endpoint to send data to even if we wanted to — these are static pages on Cloudflare Pages, and the function bundles do not include a 'compute' route. You can verify in DevTools by watching the Network tab while you type into any calculator: zero outbound requests during the calculation.
We also do not store your inputs. A few tools (currency converter, unit converter) optionally remember your last-used pair via localStorage so you do not have to set it again on the next visit. That is the entirety of the persistence story. Clearing your browser storage clears all of it.
Numbers we updated this year
Currency rates are pulled from the European Central Bank reference feed daily. We do not pretend to have intraday tick data; the reference rate is the one most travelers and casual users actually need, and it is the same one banks use as a baseline before adding their margin. We list the timestamp on the converter page so you know what day's rate you are seeing.
Tax brackets for 2026 are pulled from the IRS for the United States and the Korean National Tax Service for Korea. We update them on the day they publish, which usually means December for the upcoming year. If you are planning around a year-end decision, the page banner will tell you whether you are looking at the current-year or next-year brackets.
Calorie targets use the latest Mifflin-St Jeor coefficients, unchanged in 2026 because they have held up well against more recent meta-analyses. The activity multipliers we use (1.2 sedentary up to 1.9 very active) come from the FAO/WHO/UNU report, which most clinical practice still treats as the reference.
What these calculators are not
Not medical advice. The BMI and calorie tools are first-pass scoping tools. If a calculator output prompts a real concern about your health, the next step is a professional, not a refresh of the page.
Not legal or accounting advice. The tax tools compute scenarios. They do not account for credits, deductions, or special situations that turn on facts we do not collect. Use them to set expectations before you talk to a tax preparer; do not use them to file.
Not a substitute for talking to a person. Several of these tools were built specifically because a quick number can give you the courage to start a hard conversation — about a salary negotiation, a refinance, a career change. We are happy to be the prelude to that conversation. We are not a replacement for it.