Pounds and Ounces Calculator (add, subtract, multiply, to kg)

Understanding the pounds and ounces calculator — concept and principle

This calculator adds, subtracts, multiplies and divides weights given in pounds and ounces, and shows the result in pounds and ounces, total ounces, decimal pounds, kilograms and grams. Parents tracking a baby's weight, cooks scaling recipes, and people shipping parcels use it.

Add in ounces, then regroup — with the default inputs: 8 lb 5.5 oz.

How it is calculated

Convert each weight to ounces: total = pounds × 16 + ounces
Apply the operation: A + B, A − B, A × number or A ÷ number
Split back: pounds = ⌊ounces ÷ 16⌋; ounces = remainder
Decimal pounds = ounces ÷ 16; kg = pounds × 0.45359237; g = kg × 1,000

Ounces of 16 or more are carried into pounds automatically.

The principle

Pounds and ounces form a mixed-base system with 16 ounces to the pound, so decimal arithmetic on "5.9 lb" goes wrong. Working in ounces avoids that. These are avoirdupois units, used for body weight, food and parcels; the troy ounce used for precious metals is different. Since 1959 the pound is defined as exactly 0.45359237 kg, so one ounce is exactly 28.349523125 g.

Worked example

Defaults: A = 5 lb 9 oz, B = 2 lb 12.5 oz, operation A + B.

A = 5 × 16 + 9 = 89 oz; B = 2 × 16 + 12.5 = 44.5 oz.
Sum = 133.5 oz.
133.5 ÷ 16 = 8 lb remainder 5.5 oz: 8 lb 5.5 oz.
Decimal pounds = 8.3438; kilograms = 8.34375 × 0.45359237 = 3.7847; grams = 3,784.7.

How to read the result

Use pounds and ounces for everyday reading, decimal pounds for spreadsheets and shipping systems, and grams when comparing with metric product labels. For babies, small changes matter, so weigh on the same scale at similar times.

Common mistakes

Treating 5.9 lb as 5 lb 9 oz; it is 5 lb 14.4 oz.
Adding ounces without carrying 16 into a pound.
Using troy ounces for food or body weight.
Rounding early in a series of calculations.

Keep in mind: This tool converts and adds weights only. For a baby's growth or a health question, ask a health professional.

Frequently asked questions

How many ounces are in a pound?

16 avoirdupois ounces.

How many grams is one ounce?

Exactly 28.349523125 g.

Is a troy ounce the same?

No. A troy ounce is about 31.1 g and is used for precious metals.

Sources: NIST Special Publication 811 (2008 ed.), Appendix B.9 (pound and ounce avoirdupois)

Checked 9 Nov 2026. NIST Special Publication 811 (2008 ed.), Guide for the Use of the International System of Units (SI), Appendix B.9 “Factors for units listed by kind of quantity or field of science” — pound (avoirdupois) and ounce (avoirdupois); exact value 1 lb = 0.453 592 37 kg from the 1959 international yard and pound agreement; 1 lb = 16 oz.

Your numbers