Speed converter

Model: Exact unit definitions

Accuracy: Exact to double precision: each conversion is one correctly rounded ratio of exact definitions

When to use this: Use this wherever a speed crosses between trades: an airspeed or a boat speed in knots against a groundspeed in km/h, a road limit in mph, a wind in m/s, a descent in ft/s. The geodetic rates are here too -- mm/yr and m/yr -- because plate motion and subsidence are quoted in them and belong on the same scale as everything else.

Limitations: A speed is a rate and nothing more: this cannot tell airspeed from groundspeed, true from indicated, or a speed through the water from a speed over it. Those distinctions are what the navigation and aviation tools carry. The knot here is the international one, 1,852 m per hour exactly; an old chart or a British source may mean the Admiralty knot, which is about 0.06% larger and will not announce itself.

Worked example: 100 knots in mph. Source: Exact unit definitions (NIST SP 811 Appendix B). It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.

Review: Not yet independently reviewed by a metrologist.

Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.0, core 0.1.0. See this tool in the verification report.

Checked against: 47 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, National Institute of Standards and Technology, 2008 edition. Appendix B.8, Factors for units listed alphabetically.
• Annex 5 to the Convention on International Civil Aviation: Units of Measurement to be Used in Air and Ground Operations, International Civil Aviation Organization, 5th edition. Chapter 3 and Table 3-3. Paid only.