Calculate Kinetic Energy (ft-lbs)
Use this free calculator to estimate kinetic energy in foot-pounds (ft-lbs) and joules.
Tip: Press Enter in any field to calculate instantly.
What is ft-lbs energy?
Foot-pounds (ft-lbs) is a common unit used to describe kinetic energy in the US, especially for ballistics, archery, airguns, and impact tools. It tells you how much work an object can do because of its motion.
In simple terms: higher velocity and higher mass (or weight) create higher energy.
How this ft lbs energy calculator works
Method 1: Grains + FPS (common for bullets and arrows)
Formula used:
Energy (ft-lbs) = (Weight in grains × Velocity²) ÷ 450240
This shortcut formula is standard in ballistics and is convenient when projectile weight is listed in grains.
Method 2: Pounds + FPS (general physics use)
Formula used:
Energy (ft-lbs) = (Weight in pounds × Velocity²) ÷ (2 × 32.174)
Here, 32.174 ft/s² is gravitational acceleration used to convert weight-based input into mass terms correctly.
Joule conversion
The calculator also reports joules using:
Joules = ft-lbs × 1.35581795
Step-by-step usage
- Select your input type: grains or pounds.
- Enter projectile/object weight.
- Enter velocity in fps.
- Click Calculate Energy.
- Read your result in both ft-lbs and joules.
Example calculations
Example 1: Rifle round
A 55-grain projectile at 3,200 fps:
(55 × 3200²) ÷ 450240 = 1250.98 ft-lbs
Example 2: Archery setup
A 400-grain arrow at 280 fps:
(400 × 280²) ÷ 450240 = 69.66 ft-lbs
Common mistakes to avoid
- Mixing up grains and grams (they are very different units).
- Using mph instead of fps for velocity.
- Assuming ft-lbs alone tells the full story of performance.
- Rounding too early when comparing loads.
Why energy matters
Kinetic energy can be useful for comparing setups, estimating impact potential, and tracking performance changes. That said, real-world outcomes also depend on projectile design, target characteristics, distance, drag, and shot placement.
Quick FAQ
Is more ft-lbs always better?
Not always. It depends on your application. Accuracy, consistency, and projectile behavior can matter just as much.
Can I use this for airguns and bows?
Yes. Use grains for typical pellet/arrow weight inputs and fps for measured velocity.
Is this calculator exact?
It is mathematically accurate for the formulas shown. Real-life performance can still vary with environmental and equipment factors.