How Accurate Are Online Unit Converters? (And When You Shouldn't Trust Them)

Where do conversion factors come from?

A converter that uses these sources will give you the same answer as any other converter that uses the same sources — to within the limits of floating-point arithmetic and rounding.

• NIST (National Institute of Standards and Technology) — the U.S. reference for physical constants, units, and conversion factors.
• BIPM (Bureau International des Poids et Mesures) — the international custodian of the SI system.
• ISO 80000 — the international standard for quantities and units.
• Specialized references for niche units: the CODATA recommended values for physical constants, historical reference works for ancient units, IUPAC for chemical units.

The hidden limitation: floating-point arithmetic

Computers represent numbers in binary, and not all decimal numbers can be represented exactly. 0.1 in binary is a repeating fraction, the same way 1/3 is in decimal. This causes tiny rounding errors that can accumulate.

For most everyday conversions, this is irrelevant — the error is around 10⁻¹⁵, far smaller than the precision you need. But for extreme cases (multiplying thousands of values, working at the limits of measurement precision), it can matter. UnitSwiftPro uses 64-bit floating-point, which is more than sufficient for any realistic conversion.

Rounding and significant figures

A converter that shows 12 decimal places is being misleadingly precise. Real-world measurements rarely have more than 3–4 significant figures. If you measure a length as 1.234 m, you don't actually know the length to within 1 µm — your measuring tape is only good to the nearest mm.

A good converter shows you the right number of significant figures. UnitSwiftPro's default is 6 significant figures, which is more than enough for everyday use. You can adjust it up or down depending on your needs.

When the data is just wrong

UnitSwiftPro prioritizes current standard definitions and cites the system used when working with historical or specialized units. When in doubt, the formula is shown so you can verify.

• Obscure units: a "barrel" can mean different things in oil, brewing, and produce.
• Historical units: a "Roman foot" varies depending on which century and which region.
• Specialized industry units: "board feet" in lumber, "twips" in typography, "points" in font sizing — all have specific definitions that general-purpose converters sometimes get wrong.
• Old vs. new definitions: the pound was redefined in 1959. Some online converters still use the pre-1959 factor.

When to verify with the source

For everyday conversions — recipes, school homework, casual estimates — any reputable online converter is more than accurate enough. The error is well below 0.001%, and you don't need that precision.

But there are cases where you should not rely on an online converter and should verify with the original source:

• Medical dosing. Drug doses are typically calculated using body weight, body surface area, or other patient-specific factors. A unit conversion error of even 1% could be dangerous.
• Legal and contractual. Land sales, fuel deliveries, freight weights, and other legal matters may have specific statutory conversion factors. Use those, not a general converter.
• Aerospace and large-scale engineering. A famous example: in 1999, NASA's Mars Climate Orbiter crashed because one team used metric units and another used US customary units. The error was small per calculation but added up over millions of calculations.
• Financial calculations. Currency conversions, interest calculations, and other financial math use specific decimal-precision conventions. Use a dedicated financial calculator, not a unit converter.
• Scientific research. When publishing results, you should cite the source of your conversion factors. Most journals require this.

A quick accuracy checklist

• Is the converter citing a known standard? NIST, BIPM, ISO, or a recognized industry body? Good. Some random blog post from 2009? Less good.
• Does it show the formula? A converter that shows you the math is one where the maintainer has nothing to hide.
• Does it match other sources? If Google, NIST, and UnitSwiftPro all give 1 kg = 2.20462 lb, you can trust that. If they disagree, dig deeper.
• Is the precision appropriate? A converter showing 17 decimal places is over-precise for almost any real use. 4–6 significant figures is usually right.
• Are you in a critical domain? If the answer affects safety, money, or legality, verify with the original source.

UnitSwiftPro's accuracy commitment

UnitSwiftPro sources its conversion factors from NIST, BIPM, and ISO standards. Every conversion shows the formula used. We use 64-bit floating-point arithmetic, which provides more precision than any real-world measurement can verify.

For everyday conversions, you can trust the result completely. For medical, legal, or aerospace applications, we provide a clear disclaimer and recommend verification with the original source. Read our about page for more on our editorial standards.