Mental Shortcuts for Metric and Imperial Conversions
Exact conversion factors are awkward to remember: 2.54, 1.609344, 0.45359237. A handful of approximate factors, though, are accurate enough for travel, shopping and DIY estimates, and metric prefixes turn conversions inside the metric system into moving a decimal point.
Below are the exact factors worth memorizing, shortcuts built on them with their error sizes, and a classic trick for miles and kilometers. Knowing the error tells you when an estimate is enough and when to reach for a converter.
Moving the decimal: metric prefixes
Metric units are linked by powers of ten, and a prefix tells you which power:
| Prefix | Symbol | Factor |
|---|---|---|
| mega | M | 1,000,000 |
| kilo | k | 1,000 |
| centi | c | 0.01 |
| milli | m | 0.001 |
| micro | μ | 0.000001 |
To move to a smaller unit, shift the decimal point to the right; to move to a larger one, shift it to the left. Worked example: 2.5 km is 2,500 m, which is 250,000 cm, which is 2,500,000 mm. Going up works the same way in reverse: 3,200 mm is 3.2 m, 450 g is 0.45 kg, and 0.75 L is 750 mL. The prefixes are identical for every metric unit, so you learn them once and reuse them for meters, grams, liters and bytes. Imperial units have no such pattern: there are 12 inches in a foot, 3 feet in a yard and 1,760 yards (5,280 feet) in a mile.
Exact factors worth memorizing
| Imperial | Metric |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot | 30.48 cm |
| 1 yard | 0.9144 m |
| 1 mile | 1.609344 km |
| 1 pound | 0.45359237 kg |
| 1 ounce | 28.35 g (28.349523 g exactly) |
The values for the inch, foot, yard, mile and pound are exact by definition, set by an international agreement on the yard and pound in 1959. Other everyday units, such as the ounce (one sixteenth of a pound), follow from them.
Shortcuts and their errors
| Conversion | Shortcut | Example | Exact | Error |
|---|---|---|---|---|
| Miles to km | × 1.6 | 60 mi = 96 km | 96.56 | −0.6% |
| Inches to cm | × 2.5 | 12 in = 30 cm | 30.48 | −1.6% |
| kg to lb | Double, add 10% | 70 kg = 154 lb | 154.3 | −0.2% |
| lb to kg | Halve, subtract 10% of that | 180 lb = 81 kg | 81.6 | −0.8% |
| Meters to feet | × 3, add 10% | 100 m = 330 ft | 328.1 | +0.6% |
| Ounces to grams | × 30 | 8 oz = 240 g | 226.8 | +5.8% |
| Liters to US gallons | ÷ 4 | 40 L = 10 gal | 10.57 | −5.4% |
Worked example, luggage: an airline allows 23 kg. Doubling gives 46, and adding 10% of that (4.6) gives 50.6 lb. The exact answer is 50.7 lb, so the shortcut is within 0.2%, which is plenty for deciding whether a bag is close to the limit.
Rounding the other way, km to miles by multiplying by 0.6 runs about 3.4% low, while 0.62 is within 0.3%. The liters-to-gallons rule is the roughest: it is fine for deciding whether a container is a lot bigger than another, but poor for anything that needs precision.
How much error is acceptable?
A percentage error is easy to misjudge until it is turned into a real length or weight. Take a height of 5 feet 10 inches, which is 70 inches. The shortcut of 70 × 2.5 gives 175 cm, while the exact 70 × 2.54 gives 177.8 cm, so the 1.6% error is 2.8 cm. That is fine for a rough description and too much for ordering tailored clothing or a made-to-measure shelf.
A useful rule: an error of 1% or less is safe for anything you will only look at, 5% is acceptable for comparing sizes or planning, and anything that must fit, seal, dose or bear a load needs the exact factor.
The Fibonacci trick for miles and kilometers
Consecutive numbers in the Fibonacci sequence (1, 1, 2, 3, 5, 8, 13, 21, 34, 55) have a ratio near 1.618, which is close to the 1.609 kilometers in a mile. So a Fibonacci number of miles is roughly the next Fibonacci number of kilometers:
- 5 miles ≈ 8 km (exact 8.05)
- 8 miles ≈ 13 km (exact 12.87)
- 13 miles ≈ 21 km (exact 20.92)
- 21 miles ≈ 34 km (exact 33.80)
For other distances, split the number into Fibonacci parts. Worked example: 50 miles is 34 + 13 + 3. Their kilometer partners are 55 + 21 + 5 = 81 km, against an exact 80.47 km, an error of under 1%.
When to use the exact value
- Use shortcuts to sense-check a number, plan a trip or compare sizes. Use exact factors for anything you will cut, mix, buy by the unit or send to someone else.
- Errors grow when conversions are chained. Converting twice with two rough factors lets the errors compound.
- Temperatures do not scale with one factor. See the article on Celsius, Fahrenheit and Kelvin, and the one on speed units for km/h and mph.
For exact answers the imperial to metric converter, the metric to imperial converter and the length converter cover inches through nautical miles, as well as nanometers through kilometers.
- Imperial to Metric Converter: Pick an imperial unit, type a length and choose a metric unit to see the answer immediately. All factors are the exact international definitions, with results shown to nine decimal places.
- Metric to Imperial Converter: Choose a metric unit, enter a length and pick the imperial unit you want. The converter uses exact definitions and shows the answer to up to nine decimal places.
- Length Converter: Pick a unit, type a length, choose the unit you want and read the answer. The converter covers metric, imperial and nautical units in one place.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
More articles
Acres, Hectares and Square Miles: Land Area Units Compared
Land listings and maps use acres in some places and hectares in others. See how the units relate, how to convert them exactly, and how to avoid the squared-factor mistake.
Celsius, Fahrenheit and Kelvin: Formulas and Oven Marks
Temperature scales do not share a zero, so conversion needs a multiplication and an addition. Here are the exact formulas, a quick estimate, and an oven chart.
Clothing Size Conversion: Measure Your Body, Not the Label
A US 8, UK 12 and EU 40 are often listed as equals, yet brands label garments differently. Compare your measurements with a size chart instead, converting inches to centimeters.