Baby Size Everyday Objects Calculator

What this calculator does

Find an everyday object that weighs about the same as a typical baby at your week of pregnancy. Pick your week, or enter your last period or due date. You get the closest object, how the two weights compare, and a chart for every week from 10 to 40.

Every object here has a weight set by a standard, not a guess. Coins follow the U.S. Mint's specifications, paper follows its weight grade, and water has a known weight per gallon. The baby's weight is a typical average for the week, not a measurement of your baby. Only an ultrasound by your healthcare provider measures your own baby.

Where it's used

Why this math matters outside the classroom:

• Counting coins. Students multiply 20 nickels by 5 g each to find that a dollar in nickels weighs exactly 100 g.
• Paper weight. Office staff estimate a 10-ream box of letter paper at 10 × 5 lb, which is 50 lb, before lifting it.
• Water by volume. Campers carrying 3 gallons of water know it weighs about 3 × 8.34 lb, or 25 lb.
• Comparing weights. Families divide a typical 29-week weight of 1,379 g by 1,134 g of paper to get a ratio of about 1.22.

How to use it

1. Choose how to start: the week you're in, the first day of your last period, or your due date.
2. Enter the week (10 to 40) or the date. For the date options, "Counting from" starts as today; change it to check another day.
3. Read the closest object. Below it is the ratio, the typical weight and the typical length.
4. Open the table to see the matching object for every week.

What the inputs mean

Week of pregnancy

Completed weeks since the first day of your last period. For example, 29 weeks 3 days counts as week 29.

First day of last period

The first day of bleeding of your most recent period. The calculator assumes a 28-day cycle.

Estimated due date

The date your healthcare provider gave you, or one from a due date calculator.

Counting from

The day to work out the week for. It starts as today.

Formula and method

First day of last period = due date − 280 days
Weeks pregnant = days since that day ÷ 7
Typical weight (g) = e^(0.578 + 0.332 × W − 0.00354 × W²)
Ratio = typical weight ÷ object weight
Fair match when the ratio is between 0.67 and 1.5

W is the number of completed weeks of pregnancy. The letter e is a fixed number, about 2.718. The weight formula comes from Hadlock and colleagues (1991) and gives the middle weight for each week.

The ratio compares the two weights. A ratio of 1 means the same weight, 1.2 means the baby is 20% heavier, and 0.8 means 20% lighter. The calculator picks the object with the ratio nearest to 1, judged by proportion.

If even the nearest object were more than 1.5 times heavier or lighter, the page would say there's no close match instead.

Worked example

Your due date is March 1, 2027. How does a typical baby compare on December 14, 2026?

1. First day of the last period: Mar 1, 2027 − 280 days = May 25, 2026.
2. Days so far: Dec 14, 2026 − May 25, 2026 = 203 days. 203 ÷ 7 = 29 weeks, 0 days.
3. Weight formula: 0.578 + 0.332 × 29 − 0.00354 × 29² = 7.2289, and e^7.2289 = 1,379 g.
4. The nearest object is a half ream of printer paper (250 sheets) at 1,133.98 g. Ratio: 1,379 ÷ 1,133.98 = 1.22.
5. 1.22 is between 0.67 and 1.5, so it's a fair match.

Result: At 29 weeks, a typical baby weighs about 1,379 g (3 lb 0.6 oz), about 1.2 times the weight of a half ream of printer paper.

Understanding the result

Coins make good weights because the U.S. Mint sets each one exactly. A penny weighs 2.5 g and a nickel weighs 5 g. So a dollar in nickels (20 coins) weighs 100 g, and a dollar in pennies (100 coins) weighs 250 g.

Paper is sold by a weight grade. "20 lb" printer paper means 500 large sheets, each 17 by 22 inches, weigh 20 pounds. A letter-size sheet is a quarter of that large sheet. So a 500-sheet ream of letter paper weighs 5 pounds, and each sheet weighs about 4.5 g.

Water weighs about 8.34 pounds per gallon. Cold water is a touch heavier and warm water a touch lighter, but the change is tiny. A quart is a quarter of that, and a half gallon is half.

For a sense of length, a payment card is a handy ruler. Its size is fixed by an international standard at 85.60 by 53.98 mm. At 12 weeks, a typical baby's head-to-bottom length of 5.6 cm is about the card's short side.

Assumptions and limitations

• Object weights follow their standards: U.S. Mint coin weights, 20 lb bond paper, 8.34 lb per gallon of water, and the USDA weight of a 12-ounce can of soda (370 g, the drink only). Containers are not included.
• The week is counted from the first day of the last period, with a due date 280 days after that day. Sizes are given for whole weeks.
• From 41 weeks to 42 weeks 6 days, the 40-week figures are shown, since the weight formula stops at 40 weeks.
• These are averages, not a measurement or a prediction for any one baby. Twins and other multiples are often smaller.
• The match uses weight only, not size or shape.
• Real paper and coins vary a little within their makers' allowed limits.
• It is not a medical tool. It can't detect any problem, and it doesn't replace prenatal care.

Frequently asked questions

How this calculator is checked

Method: Typical weight: Hadlock, Harrist & Martinez-Poyer, Radiology 1991;181:129–33, 50th-percentile equation (10–40 weeks), with lengths and dating as in src/lib/health/fetal-size.ts. Object weights: U.S. Mint coin specifications (cent 2.500 g, nickel 5.000 g); 20 lb bond basis weight (500 sheets of 17 × 22 in = 20 lb); 8.34 lb per US gallon of water; USDA FoodData Central SR Legacy 174852 for a 12 fl oz soda. Matching: closest weight on a ratio scale, with a 1.5× limit. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-26. Found a problem? Tell us.

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