Free Online Calculators
Quick, accurate, ad-free everyday calculators.
From splitting a restaurant bill to figuring out exactly how far along a pregnancy is, SwiftTooly's calculators give you a clear answer in a single screen. They run on the math built into your browser — no servers, no accounts, no analytics on the numbers you type. Bookmark any one of them and you have an instant answer next time you need it.
8 Calculators
Why use SwiftTooly's Calculators
Honest math
Documented formulas, no surprise add-ons, no marketing layer on top of the result.
Private inputs
Your weight, age, due date and tip amount stay on your device — we never see them.
Mobile-first
Large tap targets and number inputs make these usable one-handed on a phone.
Zero ads in the way
A clean interface focused on the answer, not on keeping you on the page.
Calculators in your browser vs uploaded to a server
SwiftTooly (browser)
Every formula runs in a few lines of JavaScript inside the tab. You can disable the network and the calculator still works.
Typical upload-to-server tools
Many sites round-trip even simple math to a backend so they can log inputs, A/B test, or attach them to your profile.
What do these calculators leave out?
Each tool here implements a specific, well-defined piece of arithmetic, and the honest framing is that the arithmetic is the easy part — knowing what it leaves out is what makes a result usable. The percentage calculator handles the three standard questions and, importantly, distinguishes percentage change from percentage points, a confusion that produces genuinely misleading numbers in everyday reporting. The BMI calculator applies a formula devised in the 1830s for population statistics, not for individuals, and it cannot distinguish muscle from fat or account for where fat sits. The due date calculator applies Naegele's rule, which assumes a regular 28-day cycle and is superseded by first-trimester ultrasound dating. The time duration calculator handles overnight shifts and unpaid breaks and gives payroll-ready decimal hours. In each case the page itself sets out the limitations rather than presenting a number as more authoritative than it is.
Decimal hours, percentages and the two errors that recur
Two arithmetic mistakes account for a large share of everyday calculation errors and both are worth naming directly. The first is treating minutes as a decimal. Seven hours forty-five minutes is 7.75 hours, not 7.45 — minutes divide by 60 rather than by 100. Writing 7.45 on a timesheet understates the shift by eighteen minutes, and across a month of shifts that is most of a working day unpaid. Payroll systems expect decimal hours precisely because pay is hours multiplied by rate. The second is assuming percentages are reversible. A 20 percent increase followed by a 20 percent decrease does not return you to the original figure, because each percentage applies to a different base — 100 becomes 120 becomes 96. The same asymmetry means a 50 percent fall requires a 100 percent gain to recover. Successive percentage changes combine by multiplying the factors, never by adding the percentages.
Privacy in a calculator, and why it is not trivial
It is reasonable to ask why a calculator needs to run locally at all. The answer is what people type into them. A BMI calculator receives height and weight. A due date calculator receives menstrual dates. A time duration calculator receives an employment pattern. A salary converter receives income. Individually these are unremarkable; collected against an IP address and a browser fingerprint they are exactly the kind of health, employment and financial data that privacy regulation treats as sensitive. Everything on this page is computed in JavaScript inside your browser tab. No figure you enter is transmitted, logged or retained, and the tools continue to work with the network disconnected — which is the simplest way to verify the claim rather than take it on trust. That also means there is no history and no saved state. Refreshing the page clears everything, which is deliberate: a calculator that remembered your last entry would have to store it somewhere, and the entire point is that nothing is stored. Copy any result you need before navigating away.
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Frequently asked questions
Are these calculators accurate enough for real decisions?
- The math itself is exact (e.g. BMI = kg / m², Naegele's rule for due dates). They are general-purpose estimates, not professional advice — for medical or legal decisions, consult a qualified professional.
Do you store the values I type in?
- No. Inputs live only in your browser tab and are wiped the moment you close it. There is no analytics call carrying your numbers.
Why does the BMI calculator support both metric and imperial?
- Because health charts use both. We let you pick units rather than forcing a conversion that introduces rounding errors.
Can I use these calculators offline?
- After the first load, yes — they don't need a network round-trip to compute a result.