Exactly how many centimetres are in one inch?
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Answer
2.54
An inch is defined as exactly 2.54 cm, under the 1959 international agreement on the yard and pound. 2.45 is the classic swapped digit slip.
Everyday Know-How
Units, conversions and how ordinary things work
Everyday Know-How is a Bertopia flashcard pack of 60 cards: units, conversions and how ordinary things work. It covers length and distance; weight and mass; volume; in the kitchen; temperature; speed; area; electricity at home; how things work; time and calendars; numbers and units.
Sample cards
How many ounces are in an ordinary (avoirdupois) pound?
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Answer
16
An avoirdupois pound has 16 ounces. 14 pounds make a stone. The troy pound used for precious metals has 12 troy ounces.
How many teaspoons are in a US or UK tablespoon?
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Answer
3
A US or UK measuring tablespoon is 3 teaspoons, about 15 ml against 5 ml. An Australian tablespoon is 20 ml, or 4 teaspoons.
One litre is equal to how many cubic centimetres?
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Answer
1,000
A litre is one cubic decimetre, which is 1,000 cubic centimetres. So 1 ml equals 1 cubic centimetre.
At what temperature do the Celsius and Fahrenheit scales show the same number?
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Answer
Minus 40
Setting C equal to 9C/5 + 32 gives C = minus 40. So minus 40 °C is minus 40 °F.
A knot is a speed of one what per hour?
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Answer
Nautical mile
One knot is one nautical mile per hour. A nautical mile is 1,852 m, so a knot is about 1.852 km per hour.
Sources
27 sources
- NIST SP 811, Guide for the Use of the SI, Appendix B.8 and B.9 conversion factors, NIST
- NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement, NIST
- NIST, SI Units: Length, NIST
- BIPM, The International System of Units (SI Brochure), 9th edition, BIPM
- NWS El Paso, Temperature Conversion formulas, NOAA National Weather Service
- EIA, Measuring electricity, US Energy Information Administration
- OSHA 3075, Controlling Electrical Hazards, US Occupational Safety and Health Administration
- Energizer E91 AA alkaline product datasheet, Energizer
- Energizer NH15 AA NiMH product datasheet, Energizer
- FTC, Shopping for Light Bulbs, US Federal Trade Commission
- FDA, Microwave Ovens, US Food and Drug Administration
- National Center for Home Food Preservation, Ensuring Safe Canned Foods, University of Georgia, NCHFP
- NASA Space Place, Why Is the Sky Blue?, NASA
- NASA Space Place, What Causes the Seasons?, NASA
- NASA Space Place, What Is a Light-Year?, NASA
- NIST, How UTC(NIST) is related to Coordinated Universal Time, NIST
- NOAA NCEI, Magnetic Declination, NOAA National Centers for Environmental Information
- International ISBN Agency, What is an ISBN?, International ISBN Agency
- GS1 US, Guide to UPCs, GS1 US
- 16 CFR 23.3, Misuse of the terms gold and karat (FTC Jewelry Guides), US Federal Trade Commission, via Cornell LII
- CDC, How to Safely Clean and Sanitize with Bleach, US Centers for Disease Control and Prevention
- CDC, Carbon Monoxide Poisoning Basics, US Centers for Disease Control and Prevention
- CDC, Chlorine chemical fact sheet, US Centers for Disease Control and Prevention
- OpenStax Chemistry 2e, OpenStax, Rice University
- OpenStax College Physics, OpenStax, Rice University
- OpenStax Microbiology, OpenStax, Rice University
- NIST, Time and Frequency from A to Z, L, NIST
