Every digit, justified.
Search “significant figures calculator” and you’ll find dozens of tools that all do the same thing: you type in a number, and something tells you an answer. Almost none of them tell you why. None of them mention that a chemistry teacher, an AP exam grader, and a lab running an ASTM-compliant conformance test can look at the exact same number and correctly disagree about how many significant figures it has. That gap — between “here’s an answer” and “here’s the rule that produced it, and here’s when a different rule would produce a different one” — is what this project exists to close.
What’s actually different here
Every calculator on this site shows its work: not just the result, but which rule produced it. Where the correct answer genuinely depends on context — a classroom default, an AP rubric, an IB internal assessment, an ASTM E29 conformance test — we say so explicitly instead of picking one convention and quietly pretending the others don’t exist.
And when a topic connects to something bigger than a homework problem, we follow it there. Significant figures are a simplification of real measurement science — useful precisely because they’re simple, but still a simplification. A properly reported measurement uncertainty, a GUM-compliant error propagation, an ASTM conformance decision: these are the real thing sig figs are standing in for, and we think that’s worth showing rather than hiding behind a flat “here’s your answer” box.
What you’ll find here
The site is organized around one subject, covered in depth rather than one calculator bolted onto an unrelated blog:
- The core rules — every case of significant figures, including the genuinely ambiguous ones (which trailing zeros count, and why that’s a real limitation of decimal notation, not a trick question)
- Rounding — all the standard methods, and where they come from: classroom convention, ASTM E29, ISO 80000-1, NIST’s laboratory guidance
- Scientific notation, precision, and uncertainty — the layer underneath sig fig rules, for anyone who needs to go past the classroom shortcut into how uncertainty actually propagates through a calculation
- Subject-specific guides — chemistry, physics, and the other places these rules meet real data with real stakes
- How the tools behave — including the honest reasons a spreadsheet, a script, and a calculator can round the exact same number three different ways without any of them being wrong
How a page here actually gets made
A page on this site starts with primary sources, not with what other calculator sites already say. When a claim traces back to a standard or a technical document, we check it against that document directly. If something we assumed turns out not to hold up against the primary source, we fix it before it’s published — that’s not an exception, it’s the process working as intended.
The calculators are held to the same standard. Every numeric tool on this site runs on an arbitrary-precision decimal engine, not the native floating-point math most calculators quietly rely on — the same floating-point behavior responsible for the famous “0.1 + 0.2” not displaying as exactly “0.3” in most programming languages. We’d rather be a little slower to ship than fast and occasionally, silently, wrong about a number.
Pages on topics with real stakes — anywhere getting the convention wrong has a real consequence, not just a wrong homework answer — get reviewed by someone with relevant subject-matter background before they go live. We’re building that review process deliberately rather than skipping it to publish faster. If a page here doesn’t yet show a named reviewer, that’s because we haven’t found the right person for it yet, not because we decided the step didn’t matter.
What this project isn’t
This isn’t a general calculator directory that happens to include a significant figures tool. Everything here stays inside one subject — precision, rounding, and the measurement science underneath both — because going deep on one thing seemed more useful than going shallow on fifty unrelated ones.
This is a living project
New pages are still being added, and older ones get revisited as the review process catches up with them. That’s a deliberate trade-off: a smaller site we can actually stand behind, over a larger one padded out faster than it can be checked.
If you find something here that’s wrong, imprecise, or missing an important caveat, we want to know — [contact link]. Precision is the entire premise of this site; hearing about the places we haven’t gotten it quite right yet is how it stays true.