Blue Line: Force Curve | Red Dot: Your Actual Weight
Draw Length Sensitivity Analysis
How weight changes if you over/under draw:
Draw Length
Draw Weight (lbs)
Arrow Weight (gr)
What is Calculate Longbow Weight?
To calculate longbow weight accurately is to determine the actual force required to hold a longbow string at a specific draw length. Unlike modern compound bows with mechanical let-off, a traditional longbow's holding weight increases continuously as you pull the string back. This physical property is known as the "force curve."
The standard rating found on most bow limbs (e.g., 45# @ 28″) is a static measurement taken at the industry-standard draw length of 28 inches (AMO standard). However, very few archers have a draw length of exactly 28 inches. If your draw length is shorter, you are holding less weight than the rated number. If your draw length is longer, you are holding significantly more.
This calculator is essential for traditional archers, bowhunters, and competitive shooters who need to match their arrows to their bow's actual dynamic output rather than the sticker rating. Understanding how to calculate longbow weight ensures you select arrows with the correct spine (stiffness) and achieve optimal flight characteristics.
Longbow Weight Formula and Mathematical Explanation
The mathematics behind how to calculate longbow weight relies on a linear or slightly exponential extrapolation from the rated standard. While every piece of wood or fiberglass laminate behaves differently, archers use a reliable approximation formula for tuning purposes.
Standard Approximation Formula: Actual Weight = Rated Weight + ((Actual Draw Length – Rated Draw Length) × Weight Change Factor)
For most longbows and recurves, the "Weight Change Factor" is often estimated as either 2 to 3 pounds per inch or approximately 5% of the rated weight per inch.
Variable
Meaning
Typical Range
Rated Weight
The manufacturer's specified weight at 28″.
20 – 100 lbs
Draw Length Delta
Difference between your draw and 28″.
-4″ to +4″
Stacking Factor
Rate at which weight increases per inch.
2.0 – 3.5 lbs/inch
Practical Examples (Real-World Use Cases)
Example 1: The Tall Archer (Overdrawing)
Scenario: John is a hunter with a long 30-inch draw length. He buys a vintage longbow rated at 50 lbs @ 28″. He needs to calculate longbow weight to buy the right arrows.
Financial/Gear Implication: John is actually shooting 55 lbs. If he buys arrows spined for 50 lbs, they will be too weak, potentially breaking or flying erratically. He needs stiffer, heavier arrows.
Example 2: The Youth Archer (Underdrawing)
Scenario: Sarah has a draw length of 24 inches. She is using a club bow rated 35 lbs @ 28″.
Result: Sarah is only holding about 28 lbs. This is important for her coach to know so they don't over-spine her arrows, which would cause them to veer left (for a right-handed shooter).
How to Use This Calculator
Follow these steps to effectively calculate longbow weight and optimize your setup:
Enter Rated Weight: Look at the limb of your bow. Enter the number usually written after the "#" symbol.
Verify Rated Length: Leave this at 28 inches unless your bow specifically states a different standard (common in some custom bows).
Measure Your Draw Length: Draw a measuring arrow to your anchor point. Have a friend mark the arrow at the front of the shelf (pivot point) and add 1.75 inches for the AMO true draw length. Input this number.
Select GPP (Grains Per Pound): Enter your desired arrow mass ratio. 10 GPP is a standard for heavy hunting arrows; 8 GPP is common for 3D target archery.
Analyze Results: Use the "Actual Draw Weight" to select your arrow spine chart column. Use "Recommended Arrow Weight" to choose your arrow tip weight.
Key Factors That Affect Longbow Weight Results
When you calculate longbow weight, several physical and environmental factors influence the final holding force and arrow performance.
Stacking: "Stacking" occurs when the angle of the string against the limb tips becomes obtuse, causing the weight to increase exponentially rather than linearly at the end of the draw. This calculator assumes a smooth curve, but short bows stack faster.
Limb Materials: Carbon foam limbs generally have a smoother draw curve than wood-glass laminates. All-wood selfbows may lose weight if held at full draw (hysteresis).
String Material: A non-stretch string (Fast Flight) doesn't change draw weight, but it transfers energy more efficiently than Dacron (B-50), making the bow "feel" faster even at the same calculated weight.
Tiller: The balance between the top and bottom limb can affect how the weight is distributed on your fingers, though the total weight remains consistent.
Temperature: Extreme cold can make limb materials stiffer, slightly increasing draw weight, while extreme heat can soften resins in glass bows.
Scale Calibration: If you verify these results with a handheld scale, ensure the scale is calibrated. Digital luggage scales are a cheap alternative to dedicated archery scales for checking your results.
Frequently Asked Questions (FAQ)
1. Does draw weight increase evenly per inch?
Generally, yes, for the middle part of the draw (from 20″ to 28″). However, past 28 or 29 inches, many shorter bows begin to "stack," meaning the weight increases by 3, 4, or 5 pounds per inch instead of the usual 2.5.
2. What is the minimum legal weight for hunting?
Most states require a minimum draw weight of 40 or 45 lbs at your draw length. You must calculate longbow weight for your specific length to ensure you are legal. A 45# bow drawn to only 26 inches might only be 40#, which could be illegal in some jurisdictions.
3. How does brace height affect draw weight?
Twisting your string to increase brace height creates more "pre-load" on the limbs. This can slightly increase the initial draw weight and the holding weight, usually by 1-2 lbs depending on how much you twist it.
4. Why is Grains Per Pound (GPP) important?
GPP protects your bow. If an arrow is too light (under 7-8 GPP for wood bows), it absorbs too little energy, causing the excess energy to shock the bow limbs. This can result in limb failure (cracking). Heavy arrows absorb more energy, making the bow quieter and safer.
5. Can I reduce my longbow's weight?
Not easily. Unlike compound bows with limb bolts, longbow weight is fixed. A bowyer can sometimes sand the limbs to reduce weight by 2-3 lbs, but this removes finish and voids warranties.
6. Does this calculator work for Recurves?
Yes. The physics for calculating draw weight changes based on draw length is nearly identical for recurves and longbows, though recurves often store more total energy due to limb geometry.
7. What is AMO Length vs. True Draw Length?
True Draw Length is the distance from the nock groove to the pivot point of the grip. AMO Draw Length is True Draw Length + 1.75 inches (roughly to the front of the bow). This calculator assumes AMO standard inputs.
8. How accurate is this calculator compared to a scale?
This tool provides a mathematical ideal based on standard limb modulus. Actual wood density and manufacturing tolerances can vary by +/- 1-2 lbs. Always verify with a scale for competition setups.