What this calculator does
See which sports ball weighs about the same as a typical baby at your week of pregnancy. Enter your week, or let the calculator work it out from your last period or due date. You get the closest ball, how the two weights compare, and a chart for every week from 10 to 40.
The baby's weight is a typical average for the week, not a measurement of your baby. Babies vary a lot, and only an ultrasound by your healthcare provider measures your own baby. The ball weights come from each sport's official rules, so every one is a number you can look up.
Where it's used
Why this math matters outside the classroom:
- Finding a midpoint. Students average a tennis ball's limits of 56.0 and 59.4 g: 115.4 ÷ 2 = 57.7 g.
- Pounds to grams. Bowlers convert a 6-pound ball to grams by multiplying 6 by 453.59, which gives about 2,722 g.
- Comparing weights. Dividing 2,813 g by 2,721.6 g gives 1.03, so a typical 36-week baby weighs about as much as a 6-pound bowling ball.
- Checking equipment. Referees weigh a soccer ball before a match to make sure it falls between 410 and 450 g.
How to use it
- Choose how to start: the week you're in, the first day of your last period, or your due date.
- Enter the week (10 to 40) or the date. For the date options, "Counting from" starts as today; change it to check another day.
- Read the closest ball. Below it you'll see the ratio, the baby's typical weight and its typical length.
- Open the table to watch the match move from a golf ball at week 10 to a bowling ball at week 40.
What the inputs mean
- Week of pregnancy
- Completed weeks since the first day of your last period. For example, 36 weeks 2 days counts as week 36.
- 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
Typical weight (g) = e^(0.578 + 0.332 × W − 0.00354 × W²) Ball weight = midpoint of the allowed range = (lightest + heaviest) ÷ 2 Ratio = baby's typical weight ÷ ball's weight Fair match when the ratio is between 0.67 and 1.5
W is the number of completed weeks of pregnancy, and e is a fixed number, about 2.718. The baby's weight comes from a formula by Hadlock and colleagues (1991). It gives the middle weight for each week, so half of babies weigh more and half weigh less.
Most rule books allow a small weight range rather than one exact figure. A tennis ball, for example, must weigh 56.0 to 59.4 g. This page uses the midpoint of each range, here 57.7 g.
A few rules set only a limit. A golf ball may weigh no more than 45.93 g, and a duckpin bowling ball no more than 3 lb 12 oz. With no lowest weight to average, the page uses the limit itself.
Tenpin bowling balls have a top limit of 16 lb and no lowest weight. They are sold by the pound, so the page lists 6, 8, 10, 12, 14 and 16 lb balls at those weights.
Worked example
You are 36 weeks pregnant. Which ball weighs about the same as a typical baby?
- Weight formula: 0.578 + 0.332 × 36 − 0.00354 × 36² = 7.9422.
- Undo the log: e^7.9422 = 2,813 g, about 6 lb 3.2 oz.
- The nearest ball is a 6-pound bowling ball: 6 × 453.59237 = 2,721.6 g.
- Ratio: 2,813 ÷ 2,721.6 = 1.03, which is between 0.67 and 1.5, so it's a fair match.
Result: At 36 weeks, a typical baby weighs about 2,813 g (6 lb 3.2 oz), about the weight of a 6-pound bowling ball.
Understanding the result
A ratio near 1 means the two weigh about the same. A ratio of 1.3 means the baby is 30% heavier than the ball, and 0.8 means it is 20% lighter.
Size can fool you here. A volleyball is much bigger than a baseball, yet it weighs less than two baseballs. A volleyball is mostly air inside a thin shell, and a baseball is packed solid. This page compares weight only, so a big, light ball can match a small baby.
Some sports are less familiar. A bocce pallino is the small target ball that players aim at, and a duckpin ball is a small bowling ball with no finger holes. They fill weight gaps that the best-known balls leave open.
Assumptions and limitations
- Ball weights come from each sport's governing rules, using the midpoint of the allowed range or a single limit where that is all the rules give.
- 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.
- Real balls vary within their allowed range, and youth or practice balls can be lighter.
- The 10, 12, 14 and 16 lb bowling balls are heavier than any typical baby before birth, so they never come up as a match.
- It is not a medical tool. It can't detect any problem, and it doesn't replace prenatal care.
Frequently asked questions
Why does the page use the middle of each weight range?
Rule books allow a range so makers have a little room. A soccer ball, for example, may weigh 410 to 450 g. The midpoint, 430 g, is the fairest single figure, since a real ball could sit anywhere in that range.
Why is a bowling ball the match at the end of pregnancy?
Few balls weigh more than a basketball's 600 g. A typical baby passes 2 kg around week 33. A 6-pound bowling ball (about 2.7 kg) and an 8-pound one (about 3.6 kg) are the closest balls from then on.
What is a bocce pallino?
It is the small ball players try to land their bocce balls near. The international rules set it at 90 g, give or take 7 g. That makes it a close match around week 14, when a typical baby weighs about 93 g.
Is my baby too big or too small if my scan shows a different weight?
This page can't tell you that. The weight here is an average, and healthy babies vary widely around it. Ultrasound weights are estimates too. Your healthcare provider looks at your scans and history together, so ask them about your baby's growth.
Are the dates I enter stored or shared?
No. The math runs in your browser, and this calculator doesn't create a shareable link with your dates.
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. Ball weights: ITTF Laws 2.3.2; R&A/USGA Rules of Equipment 4.2; ITF Rules of Tennis Appendix I; CBI bocce rules Article 6; Official Baseball Rules 3.01; WBSC fastpitch softball rules Appendix 3; FIVB rules 3.1; IFAB Laws of the Game Law 2; FIBA equipment rules; NDBC rule 201; USBC equipment specifications. Each is cited by URL in items.ts. Matching: closest weight on a ratio scale, with a 1.5× limit for a fair match (src/lib/health/themed-baby-size.ts). 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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