One mental model for every conversion
Unit conversion feels like dozens of separate problems — feet to metres, ounces to grams, mph to km/h — but underneath, every conversion in a category uses the same trick. Each unit is defined by a single number: its size relative to one chosen base unit. For length, the base is the metre. A kilometre’s factor is 1,000 (a kilometre is 1,000 metres); a foot’s factor is 0.3048. Once you know each unit’s factor, any-to-any conversion is two steps:
base value = input × (source unit factor)
result = base value ÷ (target unit factor)To turn 5 feet into metres: 5 × 0.3048 = 1.524 metres. To then express that in inches: 1.524 ÷ 0.0254 = 60 inches. The Unit Converter does exactly this internally across five categories, which is why it can show you a full table of every unit at once rather than just the single pair you asked about — it’s already computed the base value.
The five categories and their units
The converter groups units into families because you can only convert within a family — metres to miles makes sense, metres to kilograms does not.
- Length — mm, cm, m, km, inch, foot, yard, mile. Base unit: metre.
- Weight (mass) — mg, g, kg, metric ton, oz, lb, stone. Base unit: kilogram.
- Volume — ml, litre, fl oz, cup, pint, quart, gallon, m³. Base unit: litre.
- Area — mm², cm², m², km², sq ft, sq yard, acre, hectare. Base unit: square metre.
- Speed — m/s, km/h, mph, knot, ft/s. Base unit: metre per second.
Pick the category tab that matches what you’re measuring, enter a value, choose your From and To units, and the result appears instantly alongside a table of all the other units in that family. If you mostly work with distances, the dedicated Length Converter guide drills into the metre-foot-inch relationships in more detail.
Metric vs imperial: where conversions go wrong
Most conversion errors aren’t arithmetic mistakes — they’re definition mistakes. A few traps to know:
The two gallons
The word “gallon” means different things on different continents. The US gallon is 3.785 litres. The imperial (UK) gallon is 4.546 litres — roughly 20% larger. A recipe, fuel-economy figure, or shipping quote that doesn’t specify which gallon can be off by a fifth. This tool uses the US gallon; if your source is British, account for the difference.
Weight versus mass
In everyday use we say something “weighs 5 kg,” but a kilogram is technically a unit of mass, while weight is a force. On Earth the distinction rarely matters and the converter treats kg, lb, and stone as interchangeable mass units — which is what you want for cooking, shipping, and body weight.
Ounces are overloaded
A fluid ounce (volume) and an ounce (weight) are completely different quantities that share a name. They live in different category tabs for exactly this reason. Make sure you’re on the Volume tab for fl oz and the Weight tab for the mass ounce.
Area scales as the square
Doubling a length quadruples the area. That’s why a square metre is not “100 square centimetres” — it’s 10,000, because 100 cm × 100 cm. Linear intuition fails for area, which is precisely why a converter that does the squaring for you prevents a class of silent errors.
Precision and rounding
The converter shows up to 8 significant figures because many conversion factors are long or exact-but-ugly: 1 inch is exactly 0.0254 m, 1 mile is exactly 1.609344 km. Showing only two decimals would discard real precision for scientific or engineering work. For most practical jobs you’ll round the result yourself — to the nearest whole unit for cooking, a couple of decimals for construction. The tool gives you the full-precision number so you decide where to round, rather than rounding for you and compounding error across a chain of conversions.
Practical workflows
Cooking from a foreign recipe. Switch to the Volume tab to turn cups and fluid ounces into millilitres and litres, then the Weight tab for ounces and pounds into grams. Watch the gallon definition if a recipe scales up.
Construction and land. The Area tab handles the jump between sq ft, sq yards, acres, and hectares — the units that appear on property listings and flooring quotes. Length covers running measurements in feet and metres.
Shipping and logistics. Convert package weights between kg, lb, and metric tons, and dimensions between cm and inches, to match a carrier’s required units.
Study and travel. Speed conversions (mph to km/h to knots) cover physics homework, road signs abroad, and sailing or aviation figures.
Handy conversion factors worth memorising
You don’t need to memorise factors when the Unit Converter is open, but a handful of anchor values let you sanity-check a result at a glance — and catch the occasional fat-fingered input:
| From | To | Multiply by |
|---|---|---|
| inch | cm | 2.54 (exact) |
| foot | metre | 0.3048 (exact) |
| mile | km | 1.609344 (exact) |
| pound | kg | 0.453592 |
| ounce (mass) | gram | 28.3495 |
| US gallon | litre | 3.785 |
| acre | hectare | 0.404686 |
| mph | km/h | 1.60934 |
Notice how many of these are exact by definition — the inch, foot, and mile were officially redefined in terms of the metre, which is why those factors never have a ”≈” in front of them. That’s also why the converter can show 8 significant figures honestly: it isn’t rounding an approximation, it’s carrying an exact ratio.
Why a converter beats a search-engine answer
It’s tempting to just type “5 feet in cm” into a search bar. That works for one-off lookups, but a dedicated converter wins for real work in three ways. First, it shows every unit in the family at once, so when you convert 5 feet you simultaneously see the value in metres, inches, yards, and miles — useful when you’re not sure which unit your audience expects. Second, it keeps you in one category, which prevents the nonsensical cross-family conversions a generic search box will sometimes attempt. Third, it preserves full precision that you can round yourself, rather than handing you a pre-rounded figure that compounds error if you chain conversions. For repeated or precision-sensitive work, those three advantages add up fast.
Putting it together
The whole of unit conversion reduces to one idea: every unit is just a multiple of a base unit, and converting means routing through that base. You don’t have to memorise factors or worry about which direction to multiply — that’s the converter’s job. Your job is to pick the right category, watch for the genuine traps (the two gallons, area squaring, volume-versus-weight ounces), and round to the precision your task actually needs.
Keep the Unit Converter handy, lean on its all-units table to sanity-check a result against neighbouring units, and conversions stop being a source of error and become a non-event.