Kinetic energy versus momentum
Momentum is arrow weight multiplied by speed, shown here in slug-ft/s. Where kinetic energy rewards speed, momentum rewards weight. Some bowhunters pay more attention to momentum when thinking about penetration on large animals, while others lean on kinetic energy. Both are simple physical calculations, and neither one is a measurement of how deep an arrow will go. Broadhead design, shaft stiffness and where the arrow hits all change the result.
If you want a deeper comparison with reference tables, see our guide to kinetic energy and FOC for bowhunters.
What the historical kinetic energy guidelines say
The scale on the calculator uses four classes you will see repeated across archery sites: under 25 ft-lb for small game, 25 to 41 for medium game such as deer, 42 to 65 for large game such as elk and black bear, and over 65 for the toughest game. They are commonly attributed to Easton. We could not trace them to a current Easton page, so we present them as historical reference only.
The calculator classifies the one-decimal number it shows you. A result of 24.99 ft-lb displays as 25.0 and falls in the medium class. That is a choice we made so the label always matches the number on screen.
Where FOC and point weight fit in
Front of center, or FOC, describes how far the arrow's balance point sits ahead of its middle. It does not change kinetic energy by itself, because the energy depends only on total weight and speed. Where FOC matters is flight. Easton suggests roughly 10 to 15 percent for hunting arrows, and heavier points, inserts and broadheads move the number up.
The two ideas connect through weight. Swapping a 100 grain point for a 125 grain point raises FOC, but it also adds 25 grains to the arrow, which lowers speed a little and raises momentum. The "Compare points" section above shows that trade-off for your own parts. For a dedicated balance check, use the FOC calculator.
The estimated FOC in this tool works from part weights and assumes where each part sits along the arrow. Those positions are approximations, so for the most accurate number, balance the finished arrow and enter the measured balance point instead.
Getting a speed you can trust
The best source for arrow speed is a chronograph reading of your actual arrow from your actual bow. Bow makers publish IBO speed ratings, but those are measured with a 70 lb draw weight, a 30 inch draw length and a 350 grain arrow. Your setup is probably different, and real-world speeds are often lower than the advertised figure.
If you do not have a chronograph, the estimator above applies common rules of thumb to your IBO rating, draw weight, draw length and arrow weight. It is a rough guide and it ignores string accessories, cam efficiency and tuning, so treat the result as an estimate. When your setup sits far from the reference conditions, the calculator says so. If you are building a sight tape, a measured speed is far better than a guess, and you can verify your marks with our sight tape verification guide or generate a new tape in the sight tape generator.
Minimum arrow weight for your bow
Many compound bow manufacturers specify a minimum arrow weight of 5 grains per pound of peak draw weight. A 70 lb bow would need an arrow of at least 350 grains, counting the point, insert, vanes, nock and any wrap. Shooting lighter arrows can damage the bow and may void a warranty. The calculator shows grains per pound for your arrow and warns when it drops below 5. Your own bow's manual has the exact figure that applies to you, and most bowhunters shoot comfortably above the minimum.
For more on how speed, weight and draw weight work together, read why arrow speed matters more than draw weight, or see our bowhunting beginner mistakes and gear guide for a first-hand account of changing arrows.
Frequently asked questions
How do you calculate arrow kinetic energy?
Multiply the arrow's total weight in grains by its speed in feet per second, squared, then divide by 450,240. The result is in foot-pounds. For example, a 400 grain arrow at 260 fps carries about 60.1 ft-lb.
How much kinetic energy do I need to hunt deer?
There is no single scientific or legal number. Historical guidelines commonly attributed to Easton list 25 to 41 ft-lb for medium game such as deer, and many hunters choose setups that exceed that. Some places set their own minimums, so check your local regulations. Broadhead sharpness, arrow construction and shot placement matter as much as energy.
What is the difference between kinetic energy and momentum?
Kinetic energy grows with the square of speed, so it rewards fast arrows. Momentum grows in direct proportion to speed, so it rewards heavy arrows. They answer different questions, and neither one alone predicts penetration.
Does FOC change kinetic energy?
No. Kinetic energy depends only on total arrow weight and speed. FOC describes where the weight sits along the arrow and affects how the arrow flies, but a heavier point also raises total weight, which does change the numbers.
Is 5 grains per pound of draw weight enough?
It is the minimum many compound bow makers specify, and shooting lighter can damage the bow. It is a floor for bow safety, not a hunting recommendation. Check your own bow's manual for its exact minimum.
How accurate is the speed estimate?
It is a rule of thumb built around the IBO reference of 70 lb, 30 in and a 350 grain arrow. It ignores string accessories, cam efficiency and tuning, and it gets less reliable far from those conditions. A chronograph reading is always better.
Turn your speed into a sight tape
Once you know how fast your arrow really flies, the Sight Tape Gen builds tape marks from your own setup in a few minutes. You can also solve unusual shots with the Shot Solver.
Open the sight tape generator