Weight

Seven units, and one distinction that this converter cannot make for you: everything here is mass. Kilograms and pounds as used on a bathroom scale, a shipping manifest or a recipe are masses. Weight is a force, and it changes with gravity while mass does not — a 70 kg person still masses 70 kg on the Moon, but weighs about a sixth as much there.

The converter opens on 1 pound, which is 0.45359237 kg exactly. That is a definition, not a measurement: the same 1959 International Yard and Pound Agreement that fixed the inch at 25.4 mm fixed the avoirdupois pound at that value, and the ounce and stone follow from it.

Stones are here because the UK and Ireland still describe body weight that way, in a two-part form the boxes cannot show directly: 11 st 3 lb has to be entered as 157 lb.

Mass, weight, and the units that quietly mean force

The confusion is baked into the pound, which does duty as both. Engineering texts split it into the pound-mass (lbm) and the pound-force (lbf), where one lbf is the force standard gravity of 9.80665 m/s² exerts on one lbm. To keep F = ma consistent in imperial units you either carry a conversion constant or switch the mass unit to the slug, which is 32.174 lbm or 14.5939 kg. The metric system avoids the whole problem by naming the force separately: the newton.

Where this actually matters: a spring scale measures force and is calibrated for Earth’s gravity, so it reads low at altitude and low near the equator, by a few parts in ten thousand. A balance compares masses and is unaffected. Load cells in freight scales are force sensors with a local-gravity correction applied at installation, which is why a certified scale cannot simply be relocated across latitudes without recalibration.

This converter has no gravity term anywhere. If your source figure is in newtons, pounds-force or kilogram-force, divide out the gravity before converting.

Two systems, seven units

Avoirdupois. Sixteen ounces to the pound, fourteen pounds to the stone. This is the system behind supermarket labels, body weight and freight in the US and UK. It is not the only pound in commercial use, which is the point of the troy comparison below.

Metric. Milligram, gram, kilogram, tonne, each a factor of a thousand apart. The tonne is 1,000 kg and is written t; it is a permitted non-SI unit rather than an SI one.

The kilogram no longer has a physical prototype. On 20 May 2019 the SI redefined it in terms of the Planck constant, fixed at exactly 6.62607015 × 10^−34 J s. The platinum-iridium cylinder held near Paris since 1889 was retired after drifting roughly 50 micrograms against its official copies over a century — a mass standard that changed value is precisely what a definition should not do.

Exact factors, and what the rounding here costs

• 1 lb = 0.45359237 kg (exact)
• 1 oz = 28.349523125 g (exact, one sixteenth of a pound)
• 1 stone = 6.35029318 kg (exact, fourteen pounds)
• 1 tonne = 1,000 kg = 2,204.6226 lb

This converter stores those factors truncated — 0.453592 for the pound, 6.35029 for the stone — which is an error of about eight parts in ten million. On a 1,000 kg pallet that is under a gram; on a single bathroom-scale reading it is invisible. It would matter for calibrating laboratory weights, which is work for a certified balance and a set of reference masses, not a web page.

Two reciprocals worth carrying: one kilogram is 2.2046 lb, and one pound is 453.59 g. The common shortcut of "multiply kilos by 2.2" runs 0.2% low, which is nearly half a pound on a 100 kg figure — 220.0 lb against a true 220.46 lb, a gap of 0.46 lb or 0.21 kg.

Three different pounds, and three different tons

The troy ounce is about 9.7% heavier than the grocery ounce, so a "one ounce" gold coin contains 31.1 g, not 28.35. Yet a troy pound is lighter than an avoirdupois pound, because it contains only twelve ounces to the other’s sixteen. This converter uses avoirdupois throughout; bullion figures need the troy factor applied separately.

The three tons differ by up to 12%, which on a single shipping container is more than a tonne of cargo. Contracts written in "tons" without qualification have gone to arbitration over exactly this.

A body weight, a suitcase, and a cup of flour

Stones to kilograms. 11 st 3 lb is 11 × 14 + 3 = 157 lb, which is 71.21 kg. Enter the pounds, read the kilograms. Going the other way, 71.2 kg is 156.97 lb, which is 11 st 2.97 lb — the fractional pound is why stone readings are usually rounded.

The airline limit that is not what it says. Most carriers set checked baggage at 23 kg and print it as "50 lb". Twenty-three kilograms is actually 50.706 lb, so a bag weighed on a pounds-calibrated scale at 50.5 lb passes the kilogram limit and fails the printed one. At the margin, weigh in whichever unit the airline’s own scale reads.

Where this converter cannot help. A recipe asking for one cup of all-purpose flour is asking for a volume, and that volume weighs anywhere from about 120 g spooned and levelled to 145 g scooped straight from the bag. No mass conversion factor bridges a volume to a weight without a density — see the Volume Converter for why baking by weight is the fix.

Shipping tiers, gym plates, and body composition

Courier pricing rounds up into bands, so the conversion decides the price: a parcel at 2.05 kg is 4.52 lb and falls into a 5 lb band, while 2.2 kg is 4.85 lb and still fits it. Knowing where the band edges sit in your own unit is worth more than four decimal places of accuracy.

Barbell plates are a good example of mixed-system drift. A 45 lb plate is 20.41 kg and a 20 kg plate is 44.09 lb, a difference of 0.9 lb each. Load four of them and a bar you are calling 225 lb is about 3.6 lb light. Competition sets are metric; most American commercial gyms are not.

For body weight specifically, the figure is an input to other calculations more often than an end in itself — see the BMI Calculator, which takes kilograms or pounds directly and has its own caveats about what the resulting number does and does not tell you.

Frequently asked questions

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