Estimate bow draw weight at your draw length from a 28-inch rated weight and gain per inch, or find proper draw length from wingspan.
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Bow Draw Weight Formula
The calculator has two main functions: estimating the draw weight of a traditional bow at your draw length, and estimating a proper draw length from wingspan or height.
Estimated draw weight at your draw length
BDW = RDW + (DL - 28) * G
- BDW = estimated bow draw weight at your draw length
- RDW = rated draw weight of the bow, usually measured at 28 inches
- DL = your draw length in inches
- G = estimated draw weight gain per inch, in pounds-force per inch
- 28 = standard reference draw length in inches for many traditional bows
This formula adjusts the bow’s rated draw weight up or down based on how far your draw length is from 28 inches. A longer draw length increases the estimated draw weight. A shorter draw length decreases it.
Draw length from wingspan, common method
DL = WS / 2.5
- DL = estimated draw length
- WS = wingspan, measured fingertip to fingertip
This method gives a common starting estimate for draw length using wingspan.
Draw length from wingspan, alternate method
DL = (WS - 15) / 2
- DL = estimated draw length
- WS = wingspan, measured fingertip to fingertip
- 15 = adjustment value used by this alternate estimate
If wingspan is not entered, the calculator can use height as a fallback measurement. Wingspan is preferred because it is more directly related to arm length and shooting posture.
Typical Draw Weight Gain Per Inch
Traditional bows usually gain weight as you draw past the rated length. The exact amount depends on the bow design, limb shape, and whether the bow starts to stack near the end of the draw.
| Setting | Gain per inch | When it is useful |
|---|---|---|
| Light | 2.0 lbf/in | A mild-drawing bow or a conservative estimate |
| Typical | 2.5 lbf/in | A general estimate for many traditional bows |
| Stacks | 3.0 lbf/in | A bow that feels noticeably heavier near full draw |
Common Draw Length Reference Values
| Wingspan | Wingspan ÷ 2.5 | (Wingspan − 15) ÷ 2 |
|---|---|---|
| 64 in | 25.6 in | 24.5 in |
| 68 in | 27.2 in | 26.5 in |
| 72 in | 28.8 in | 28.5 in |
| 76 in | 30.4 in | 30.5 in |
Example Calculations
Example 1: Estimate draw weight at a longer draw length
You have a bow rated at 40 lbf at 28 inches. Your draw length is 29.5 inches, and you use the typical gain of 2.5 lbf/in.
BDW = 40 + (29.5 - 28) * 2.5
BDW = 40 + 1.5 * 2.5 = 43.75 lbf
The estimated draw weight is 43.75 lbf.
Example 2: Estimate draw length from wingspan
Your wingspan is 70 inches. Using the common wingspan divided by 2.5 method:
DL = 70 / 2.5
DL = 28 in
The estimated proper draw length is 28 inches.
FAQ
Why are many bows rated at 28 inches?
Many traditional bows are marked with a draw weight measured at a 28-inch draw length. For example, a bow marked 40# @ 28" means it takes about 40 pounds of force to hold the bow at 28 inches of draw. If your draw length is not 28 inches, the actual weight you feel will usually be different.
Is this calculator accurate for compound bows?
This calculator is mainly for traditional bows, such as recurve bows and longbows. Compound bows have cams, let-off, draw stops, and peak weight behavior that do not follow the same simple pounds-per-inch pattern. For a compound bow, use the manufacturer’s specifications or measure draw weight with a bow scale.
Should you choose 2.0, 2.5, or 3.0 lbf per inch?
Use 2.5 lbf/in if you need a general estimate. Use 2.0 lbf/in if the bow draws smoothly or you want a more conservative estimate. Use 3.0 lbf/in if the bow stacks, meaning it gets noticeably harder to pull near full draw. The best way to confirm the result is to measure the bow with a draw scale at your actual draw length.
