Archery Bow Let-Off Calculator
Created by: Ethan Brooks
Last updated:
Calculate compound bow let-off from measured peak and holding forces, with force reduction, holding percentage and a comparison chart.
Archery Bow Let-Off Calculator
ArcheryMeasured peak and holding force • same bow setup
What does this calculator measure?
A bow let-off calculator compares measured peak draw force with measured holding force from the same compound setup. It subtracts holding force from peak force, divides the difference by peak force and expresses the result as a percentage. A 60-pound-force peak and a 15-pound-force hold produce 75% let-off: the holding force is one quarter of the peak, and the reduction is three quarters of it.
Peak and holding readings describe different positions in the draw cycle. Peak is the highest measured force, while holding is the reading at the chosen full-draw measurement position. Both must refer to the same configuration and unit. Combining a catalog peak, a reading from another setup and an assumed module percentage would produce an arithmetic answer without a consistent measurement basis. Replace the illustrative starting values with your own corresponding readings.
The output shows measured let-off, both entered forces and their absolute difference. A separate holding percentage explains how much of the peak remains. The comparison chart contains two bars, not a reconstructed draw-force curve. Two force values cannot tell the calculator the shape of that curve, the draw distance over which forces acted, or the energy transferred to an arrow during a shot.
This tool documents a measured force ratio. It does not prescribe a let-off setting, select a cam module, provide bow-adjustment instructions or determine whether a setup meets competition rules. A nominal percentage on equipment and a ratio calculated from measurements may differ for reasons this calculation cannot identify. Keep the equipment configuration, measurement position and procedure with the result so any comparison can be reviewed on a consistent basis.
How the calculation works
Let P be positive measured peak force and H the nonnegative holding force, using the same force unit. Force reduction is P minus H. Let-off is 100 multiplied by that reduction divided by P; holding percentage is 100 multiplied by H divided by P. These percentages add to 100. Because both forces have the same unit, the ratio is unchanged when both are converted between pounds-force and newtons.
Holding force above peak is rejected because it contradicts the definition of the entered peak. Equal forces produce zero reduction and zero let-off. A zero holding reading produces a mathematical 100% result but is flagged for measurement review rather than presented as a normal setup. The model preserves those boundaries, rejects blank or nonfinite inputs and rounds only the displayed values. It does not estimate measurement uncertainty or certify the readings.
Formula and symbols
Reduction = P − H; let-off (%) = 100 × (P − H) / P; holding (%) = 100 × H / P.
- P: Positive measured peak draw force (lbf or N)
- H: Measured holding force between zero and peak (same unit as P)
How to use the calculator
- Use a matching measurement pair. Record peak and holding forces from the same compound bow setup and measurement procedure.
- Choose force units. Enter both readings in pounds-force or both in newtons. Unit switching converts existing entries.
- Calculate the reduction. Submit the readings to calculate force reduction, holding percentage and let-off.
- Review exceptional readings. Recheck a zero holding reading or a holding force above peak, and keep the measurement basis with your result.
Worked examples
The 60/15 example
Suppose peak draw force is measured as 60 pounds-force and holding force as 15 pounds-force for the same setup. The reduction is 45 pounds-force. Dividing 45 by 60 gives 0.75, so let-off is 75%. Holding force is 25% of peak. The chart compares the original readings, 60 and 15, while the result table also lists the 45-pound-force difference. None of those figures describes how force varied between the two measurement positions.
The same ratio at a different force level
A second recorded setup has a 40-pound-force peak and 10-pound-force hold. Reduction is 30 pounds-force, giving the same 75% let-off and 25% holding fraction. Equal let-off percentages therefore do not imply equal holding forces. The second setup's holding reading is 5 pounds-force lower than the first example's reading, even though their percentages match. This comparison describes the supplied measurements without recommending one setup over the other.
Interpreting the boundaries
With peak and holding force both entered as 40 pounds-force, reduction and let-off are zero. With a 40-pound-force peak and a zero holding reading, the equation gives 100%, accompanied by a measurement-review notice. A holding reading of 45 with a peak of 40 is rejected rather than converted to a negative let-off, because the named peak cannot be lower than the holding observation.
Practical applications
- Record the ratio from a pair of measured forces. Keep the peak and holding values together with the result so another reader can reproduce the arithmetic. A percentage without the underlying readings does not reveal the actual forces involved in that record.
- Distinguish absolute holding force from the percentage reduction. Two setups can have identical let-off percentages but different holding forces. Reviewing the force cards alongside the percentage avoids treating those mathematically different descriptions as though they were the same measurement.
- Compare a measured ratio with a nominal equipment label as a documentation exercise. Record the exact setup and measurement procedure before interpreting a difference. The calculator can quantify the ratio, but it cannot diagnose hardware settings or explain a discrepancy by itself.
- Convert a pair of force readings consistently. Switching from pounds-force to newtons changes both displayed forces while retaining their physical values. Recalculating should give the same let-off percentage, making this a useful check on unit handling in a separate worksheet.
- Explain the relationship between holding fraction and let-off. The result states both percentages, and their sum is 100. This provides a clear way to discuss what remains at the recorded holding position without claiming a particular draw-force curve or energy profile.
- Share a measured setup record with its limitations attached. The export includes peak, holding, reduction, formula and source references. Keeping that context next to the number helps prevent a mathematical result from being mistaken for an equipment adjustment or a suitability recommendation.
Tips for consistent measurements
Use force readings from the same setup and measurement procedure. Record the full-draw position used for the holding reading, and consult the applicable manufacturer's instructions if measurements appear inconsistent. Do not combine one force in newtons with another in pounds-force. The unit selector converts both fields together; it does not change the physical meaning of peak and holding force.
Look at the absolute force values as well as the percentage. A small change in a holding reading changes the calculated ratio even when peak is unchanged, so avoid treating extra output decimals as proof of instrument precision. Recheck a zero holding reading and resolve a holding value above peak. Do not infer stored energy, arrow speed or personal holding endurance from the two entered force values.
Frequently asked questions
How is bow let-off calculated?
Subtract measured holding force from measured peak force, divide by peak force and multiply by 100. For a 60-pound-force peak and a 15-pound-force hold, the reduction is 45 pounds-force and let-off is 75%. Both readings must describe the same setup and use the same force unit for the ratio to have a consistent meaning.
What does 75% let-off mean?
It means holding force is 75% lower than the measured peak, leaving a holding fraction of 25%. It does not mean you hold 75% of peak. The absolute holding force still depends on the peak: a 60-pound-force peak gives 15 pounds-force holding at that ratio, while a 40-pound-force peak gives 10 pounds-force holding.
Can holding force be higher than peak force?
Not for a consistent pair of readings defined this way. Peak is the highest force in the measured draw cycle, so a holding reading cannot exceed it. The calculator rejects that combination and asks you to check the readings and units. It does not turn the contradiction into a negative percentage or silently swap the two values.
Why is a zero holding reading flagged?
Zero holding force gives a mathematical 100% reduction from any positive peak. The calculator can show that boundary, but the number should prompt a review of the reading and measurement procedure. It is not presented as a normal operating condition or a recommendation. The notice stays with the result and export so the qualification remains visible.
Does let-off tell me bow efficiency or arrow speed?
No. The calculation compares two force readings. Stored energy would require information about force across draw displacement, and arrow speed or transfer efficiency would require further measurements and an appropriate model. The chart is only a comparison of peak and holding values. It is not a draw-force curve and does not imply energy or speed performance.
Will changing units alter the result?
No, provided both forces are converted consistently. Multiplying peak and holding force by the same conversion factor leaves their ratio unchanged. The unit selector converts existing entries between pounds-force and newtons, clears the old result and lets you recalculate. Blanks remain blank, and the tool does not interpret kilograms as a force unit in metric mode.
Sources and measurement scope
- DeadWake Archery Innovations: What is the Let-off? — Undated online guide; What let-off actually means; 50 lbf and 12.5 lbf example. Accessed 2026-10-06. Percentage reduction from peak to holding force only. Recommendations about modules, speed, fatigue and draw-weight selection are not used.
- NIST: SP 811 Appendix B.9 — mass, force and velocity — SP 811 online Appendix B.9; Force (footnote 23); Mass and Moment of Inertia; Velocity. Accessed 2026-10-06. Grains, feet per second and pounds-force converted to SI. Exact lbf derived from 0.45359237 kg and standard gravity 9.80665 m/s².
Worked examples use illustrative measurements. These tools do not select equipment or prescribe a preferred setup. Any optional manufacturer comparison uses the reference entered by the user.