Calculate Bow Speed

Estimate your bow’s actual speed (in feet per second) based on its IBO rating and your specific setup. Enter your bow’s IBO speed and your actual draw weight, draw length, arrow weight, and any weight added to the string.

Estimated Results:

Formula Used: We start with the IBO speed and adjust for deviations from IBO standards (70lbs draw weight, 30″ draw length, 350 grain arrow, 0 grains on string). Generally: -1.5 fps per lb below 70lbs, -10 fps per inch below 30″, -1 fps per 3 grains above 350 grains, and -1 fps per 3 grains on string. Kinetic Energy (KE) = (Arrow Weight * Speed^2) / 450240. Momentum (p) = (Arrow Weight * Speed) / 225120.

Approximate Speed Loss Factors

Factor	Change	Approx. Speed Change (fps)
Draw Weight	-1 lb from 70 lbs	-1.5 to -2
Draw Length	-1 inch from 30 inches	-10 to -12
Arrow Weight	+3 grains over 350	-1
Arrow Weight	+10 grains over 350	-3 to -3.5
Added String Weight	+3 grains	-1
Added String Weight	+10 grains	-3 to -3.5

What is Bow Speed Calculation?

Bow speed calculation is the process of estimating the actual velocity at which an arrow is launched from a bow, based on its IBO (or ATA) speed rating and adjustments for the archer’s specific setup. The IBO speed is a standardized measure taken under specific conditions (typically 70 lbs draw weight, 30-inch draw length, and a 350-grain arrow), but most archers don’t shoot with these exact specifications. Therefore, it’s crucial to calculate bow speed to understand the real-world performance of their equipment.

Archers, bowhunters, and target shooters should use bow speed calculations to predict arrow trajectory, kinetic energy, and momentum, which are vital for sight pin setup, ethical hunting, and equipment tuning. A common misconception is that the IBO speed is what you’ll get out of the box; in reality, actual speed is almost always lower once you factor in your draw length, weight, arrow choice, and string accessories.

Bow Speed Calculation Formula and Mathematical Explanation

There isn’t one single, universally perfect formula to calculate bow speed because bow efficiencies vary. However, a widely used estimation method starts with the IBO speed and subtracts speed based on deviations from IBO standards:

1. Start with the bow’s IBO rated speed (S_IBO).
2. Adjust for draw weight (DW): For every pound of draw weight below 70 lbs, subtract about 1.5-2 fps. Let’s use 1.5 fps for our calculator: Speed Loss_DW = (70 – DW_actual) * 1.5 (if DW_actual < 70).
3. Adjust for draw length (DL): For every inch of draw length below 30 inches, subtract about 10-12 fps. We use 10 fps: Speed Loss_DL = (30 – DL_actual) * 10 (if DL_actual < 30).
4. Adjust for arrow weight (AW): The standard arrow for IBO is 350 grains. For every 3 grains of arrow weight above 350, subtract about 1 fps: Speed Loss_AW = (AW_actual – 350) / 3 (if AW_actual > 350).
5. Adjust for added weight on the string (SW): For every 3 grains of weight added to the string (peep, D-loop, silencers), subtract about 1 fps: Speed Loss_SW = SW_added / 3.

Estimated Actual Speed = S_IBO – Speed Loss_DW – Speed Loss_DL – Speed Loss_AW – Speed Loss_SW

Variables Table:

Variable	Meaning	Unit	Typical Range
SIBO	IBO Rated Speed	fps	300 – 370
DWactual	Actual Draw Weight	lbs	40 – 80
DLactual	Actual Draw Length	inches	26 – 32
AWactual	Actual Arrow Weight	grains	350 – 600
SWadded	Added Weight on String	grains	0 – 30

Once you have the estimated speed, you can calculate Kinetic Energy (KE) and Momentum (p).

Practical Examples (Real-World Use Cases)

Example 1: Bowhunter Setup

A bowhunter has a bow with an IBO rating of 345 fps. They shoot at 68 lbs draw weight, 29 inches draw length, with a 420-grain arrow and 15 grains of weight on the string.

• IBO Speed: 345 fps
• Draw Weight Loss: (70 – 68) * 1.5 = 3 fps
• Draw Length Loss: (30 – 29) * 10 = 10 fps
• Arrow Weight Loss: (420 – 350) / 3 = 70 / 3 ≈ 23.3 fps
• String Weight Loss: 15 / 3 = 5 fps
• Estimated Speed = 345 – 3 – 10 – 23.3 – 5 = 303.7 fps
• KE ≈ (420 * 303.7 * 303.7) / 450240 ≈ 85.9 ft-lbs

This speed and KE are excellent for most big game hunting.

Example 2: Target Archer Setup

A target archer uses a bow rated at 330 fps IBO. They shoot at 60 lbs, 28 inches, with a 360-grain arrow and 5 grains on the string.

• IBO Speed: 330 fps
• Draw Weight Loss: (70 – 60) * 1.5 = 15 fps
• Draw Length Loss: (30 – 28) * 10 = 20 fps
• Arrow Weight Loss: (360 – 350) / 3 = 10 / 3 ≈ 3.3 fps
• String Weight Loss: 5 / 3 ≈ 1.7 fps
• Estimated Speed = 330 – 15 – 20 – 3.3 – 1.7 = 290 fps
• KE ≈ (360 * 290 * 290) / 450240 ≈ 67.2 ft-lbs

This setup is optimized for accuracy over raw speed, but it’s still good to know the estimated speed for sight tapes.

How to Use This Calculate Bow Speed Calculator

1. Enter IBO Speed: Input the IBO speed rating provided by the bow manufacturer.
2. Enter Actual Draw Weight: Input your bow’s measured peak draw weight in pounds.
3. Enter Actual Draw Length: Input your measured draw length in inches.
4. Enter Arrow Weight: Input the total weight of your arrow in grains.
5. Enter Added Weight: Input the combined weight of all accessories on your bowstring in grains.
6. View Results: The calculator will instantly show the Estimated Actual Speed, Kinetic Energy, and Momentum.
7. Reset: Use the “Reset Defaults” button to go back to typical starting values.
8. Copy: Use the “Copy Results” button to copy the inputs and results for your records.

The results help you understand your bow’s performance. Higher speed means a flatter trajectory, while KE and momentum are crucial for hunting penetration. Use these values to select sight tapes or make informed decisions about your setup.

Key Factors That Affect Bow Speed Calculation Results

• IBO Rating Accuracy: Manufacturer IBO ratings are done under ideal conditions and may vary slightly.
• Bow Efficiency: Different bow designs (cam systems, limb materials) have varying efficiencies, affecting how much energy is transferred to the arrow. Our calculator uses general rules of thumb.
• String and Cable Condition: Older strings can stretch and reduce speed.
• Tuning: A well-tuned bow is more efficient and faster. Cam timing and arrow rest position matter.
• Environmental Factors: Temperature and humidity can slightly affect air density and thus arrow drag, but this is minor for speed calculation at the bow.
• Chronograph Accuracy: When comparing calculated speed to a real chronograph, the chronograph’s accuracy and lighting conditions are important.
• Draw Weight and Length Measurement: Inaccurate measurement of your actual draw weight and length will lead to errors in the calculate bow speed process.

Frequently Asked Questions (FAQ)

1. How accurate is this bow speed calculator?

It provides a good estimate based on common rules of thumb. Actual speed can vary by +/- 5-10 fps depending on bow efficiency and other factors not included in this basic calculation. A chronograph is the only way to get the exact speed.

2. What if my draw weight is over 70 lbs or draw length over 30 inches?

The calculator doesn’t add speed for going over IBO standards, as gains are less predictable and many IBO ratings are already near the bow’s peak. It focuses on speed loss from below-IBO specs. However, in reality, you would gain some speed.

3. Does arrow FOC affect speed?

Front of Center (FOC) primarily affects arrow flight and penetration, not the initial launch speed from the bow, assuming the total arrow weight remains the same.

4. Why is my chronographed speed different from the calculated speed?

Bow efficiency varies, strings stretch, tuning affects performance, and chronographs can have margins of error. The calculate bow speed tool gives an estimate.

5. How much speed do I lose with a heavier arrow?

Roughly 1 fps for every 3-5 grains of arrow weight added, but it depends on the bow’s efficiency and starting point. Our calculator uses 1 fps per 3 grains above 350.

6. Is faster always better?

Not necessarily. While speed flattens trajectory, extremely light arrows can make the bow louder, less forgiving, and may not provide enough momentum for hunting large game. A balance is often best. Check our Arrow Kinetic Energy Calculator for more on this.

7. What is a good kinetic energy for hunting?

Generally, 40-50 ft-lbs is sufficient for deer-sized game, while larger game like elk or moose requires 60-70 ft-lbs or more. We have details on our Hunting Range Calculator page.

8. How do I measure added weight on the string?

You can weigh items like peep sights, D-loops, and silencers before installation using a grain scale, or look up their specifications. You can calculate FOC here.

Related Tools and Internal Resources

• Arrow Kinetic Energy & Momentum Calculator: Calculate the KE and momentum of your arrow based on its weight and speed.
• Arrow FOC Calculator: Determine the Front of Center balance point of your arrows for better flight.
• Draw Length Calculator: Estimate your ideal draw length based on your wingspan.
• Archery Sight Tape Calculator: Generate sight tapes based on arrow speed and sight measurements.
• Archery Shot Distance Calculator: Understand arrow trajectory and drop over various distances.
• Effective Hunting Range Calculator: Estimate your maximum ethical hunting range.