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Ft. Lbs. of energy???

Discussion in 'Reloading' started by Tbone38, Apr 9, 2010.

  1. Tbone38


    Feb 7, 2010
    Is there a calculator somewhere to convert your bullet speed and bullet weight into ft. lbs of energy? If not, what is the formula? I could have sworn that I seen a link to a calculator somewhere on this site.
  2. Brian Lee

    Brian Lee Drop those nuts

    Jul 28, 2008
    Up a tree.
    Bullet weight in grains x velocity (in FPS) squared, divided by 450400.


    1200 FPS x 1200 FPS = 1440000

    180 grain bullet x 1440000 = 259200000

    259200000 / 450400 = 575.488 Foot Pounds of energy.

  3. any idea of where the constant of 450400 comes??
  4. Tbone38


    Feb 7, 2010
    Thanks man. I appreciate the info.
  5. Brian Lee

    Brian Lee Drop those nuts

    Jul 28, 2008
    Up a tree.
    You asked:
    "any idea of where the constant of 450400 comes??"

    The constant would have to vary depending on the units of measure you are using for your bullet weight and velocity, and you'll notice that when using 450400, I specified using feet per second, and grains (one pound = 7000 grains).

    First off, notice the similarity of the formula to it's more generic version popularized by Albert Einstein.

    E=M(C squared)

    E is your muzzle energy.
    M is your bullet weight.
    C is your velocity.

    So in physics they use the same formula to calculate the energy of sub atomic particles which, in Einsteins formula, were assumed to be moving at the speed of light. But Einstein's version is the unit-less version of the formula, because it still has not added a constant at the end to convert the answer into a meaningful number.

    But back to bullets:

    Suppose you were measuring your bullet velocity in miles per hour, or furlongs per fortnight, or nautical miles per hour, or whatever. Then suppose you wanted to measure your bullet weight in metric tons, or some old-time British unit like Stones.

    You could do the formula using any units you wish, as long as your constant was the correct one to convert the answer into the units you wanted your answer displayed in.

    450400 just happens to be the one that works right using Feet Per Second, Grains, and Foot/Pounds of impact energy. But you could come up with any other one you need easily.

    Lets just change out bullet velocity from FPS into Yards Per Second.

    Our original example in my first post becomes:

    400 Yards PS x 400 Yards PS = 160,000
    160,000 x 180 grains = 28,800,000

    Because we divided our velocity unit by 3 (from feet to yards) and we were using velocity squared in our formula, we need to divide our constant by 3 squared, which is 9.

    450400 / (3 squared) = 50,044.4444444 = Our new constant for use when velocity is measured in Yards Per second.

    so to finish our formula :

    28,800,000 / 50,044.4444444 = 575.488 Foot Pounds of energy.

    So as you can see, Einstein was calculating the energy present in sub atomic particles, in exactly the same way we calculate the energy in bullets hitting a target.
    Last edited: Apr 9, 2010
  6. Angry Fist

    Angry Fist Dehumanizer® Lifetime Member

    Dec 30, 2009
    Hellbilly Hill
    ^^^ Thanx, Brian! How the hell did I miss E=mc2?
  7. BobbyT


    Jul 27, 2008
    Once you know the ~450,000 constant, it's easy to do a rough calculation in your head to get in the ballpark if you just keep the most significant digit or two.

    147 gr at 1200 fps:
    1.5 * 12*12 / 450, or about 22/45 for a bit under 500 ft-lbs, say 470 (you know it isn't 50 or 5000).

    200 gr at 1100 fps:
    2 * 11*11 / 450, or 24/45 for about 550 ft-lbs.

    160 gr at 3000 fps:
    16 * 3*3 / 450, or about 150/45 for about 3300 ft-lbs.
  8. MinervaDoe


    Jan 26, 2009
    San Jose, CA
    It's very cool.
  9. BobbyT


    Jul 27, 2008
    Without going to the level of Einstein, if you remember high school physics, the formula for kinetic energy was:

    KE = MV^2 / 2

    with Mass in kg and Velocity in meters per second. So you could translate your bullet weight to kilos and your muzzle velocity to m/s, and get an answer in Joules with no need to scale it by a constant, but that would be pretty useless.

    Take each constant to go from kg to grains, m/s to fps, and joules to ft-lbs, and you get ~450,000 (or 45 or 4.5 to make the math easier, since you're going to know the rough area to be in).
  10. PEC-Memphis

    PEC-Memphis Scottish Member

    Oct 19, 2006
    Doh ?
    The best answer is that it is a resultant of the units conversion so that when using FPS for velocity and grains (for mass, although strictly speaking grains is a measure of force rather than mass) the answer is in Ft*Lbs.

    Um, not really.

    The Newtonian form for kinetic energy is E(k) = 1/2*M*V^2

    this is NOT E=M*C^2

    This formula is the MASS to ENERGY (and vice-versa) calculation.

    The "Little Boy" atomic bomb was equal to about 30,000,000 pounds of TNT. If this amount of TNT were detonated, all of the energy released would be chemical energy by an exothermic reaction - without any matter (mass of material) - being destroyed, eg. if all of the components were weighed before and after the reaction, the weight would be the same.

    However, in the "Little Boy" atomic reaction (E=M*C^2), about 1/2 of a gram of U-235 (about 1/40th of a teaspoon of U-235, weighing about the same as 1/2 a teaspoon of water) of matter was converted directly to energy. Although about 140 lbs of U-235 was needed to achieve critical mass.

    So you can see, they are really not the same at all.
  11. GioaJack

    GioaJack Conifer Jack

    Apr 14, 2009
    Conifer, CO

    We never got to finish that course in high school... the chalk kept breaking on the cave wall. :crying:

  12. OJ

    OJ Deceased Millennium Member

    The constant 450400 comes from 7000 ( grains in a pound ) multiplied by (g=32.17) by 2.
    2000 comes from 1000 ( grams in kg ) multiplied by 2
    Only first two decimal digits are used.

  13. El_Ron1


    Apr 15, 2004
    Redneck Sparta
    A fireball calculator would be useful.
  14. chemcmndr


    Aug 23, 2008
    Beavercreek, OH
    You could also use the free app for the iPod/iPhone called "Bullet Power Calc"
  15. Mystic Knight

    Mystic Knight Mystic Knight

    Aug 7, 2009
    Aiken, SC
  16. chemcmndr


    Aug 23, 2008
    Beavercreek, OH

    Well, you are correct, but let me explain it a little bit better. Both torque and energy have the same physical units of measure, but they mean two different things. The unit of energy "Joule" is defined as the amount of force (pounds) required to move an object a certain distance (foot). So, you could express energy in terms of foot-pounds (in ballistics) or you could express it in terms of Joules (S.I./metric system of measure). As a side note, the common unit of energy used in US engineering parlance is the BTU. Torque is defined as a force (pounds) applied perpendicularly to an object a distance (foot) away from a point of rotation. Foot-pounds of torque is the U.S. conventional unit. In the metric/S.I. unit system, torque is defined as a Newton-meter. Hope this helps.
  17. dbarry

    dbarry Silver Member

    Feb 15, 2010
    the Buckeye state
    Our cave wall only went to algebra II. Thank God, cuz I got a C in that! :rofl: