Child Weight from Age
This is the rule of thumb used in emergencies to set drug doses. Under one, one to five, and six and over each use a different line. Blood volume assumes 75 mL per kilogram.
Your numbers
Weight (kg)
16kg
Blood volume
1,200mL
Formula
Weight (kg) = (Age + 4) × 2 [1–5]
It stands in only when weighing is impossible. If a scale is available, use it — dose scales directly with weight.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Age (yr)Age in completed years — it shifts by one depending on the birthday. | 4 | 0 ~ 12 |
Step by step
Quick reference table
Results when only Age (yr) changes and everything else stays put.
| Age (yr) | Weight (kg) (kg) | Blood volume (mL) |
|---|---|---|
| 2 | 12 | 900 |
| 3 | 14 | 1,050 |
| 4 | 16 | 1,200 |
| 6 | 25 | 1,875 |
| 8 | 31 | 2,325 |
What each result means
| Result | At default values |
|---|---|
| Weight (kg) (kg)The second weight you are comparing against. | 16 |
| Blood volume (mL)Total blood in the body, estimated from weight. | 1,200 |
Common mistakes
It stands in only when weighing is impossible. If a scale is available, use it — dose scales directly with weight.
Glossary
- Age
- Age in completed years — it shifts by one depending on the birthday.
- Weight (kg)
- The second weight you are comparing against.
- Blood volume
- Total blood in the body, estimated from weight.
Frequently asked questions
Q. How is Child Weight from Age calculated?
Weight (kg) = (Age + 4) × 2 [1–5] — This is the rule of thumb used in emergencies to set drug doses. Under one, one to five, and six and over each use a different line. Blood volume assumes 75 mL per kilogram.
Q. Can you walk through an example?
With Age 4yr, the answer is Weight (kg) 16kg.
Q. What do I need to enter?
Enter Age. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Age (yr) moves the answer to Age (yr) 2 → Weight (kg) (kg) 12 and Age (yr) 8 → Weight (kg) (kg) 31. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
It stands in only when weighing is impossible. If a scale is available, use it — dose scales directly with weight.
Related calculators
These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.