Arrow FOC calculator
An arrow 30 inches long that balances 18 inches from the nock has a front of center of 10 percent, inside the usual target band: the balance point's lead over mid-shaft as a share of length. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Arrow length
- 30
- Balance point
- 18
- Point weight
- 125
- Foc percent
- 10
- Foc
- 0.1
- Half length
- 15
- Front of center distance
- 3
Front of center, abbreviated FOC, is the archery measurement that says how far an arrow's balance point sits ahead of its geometric midpoint, expressed as a percentage of total arrow length. This calculator applies the Easton and AMO identity: FOC percent equals one hundred times the difference between the balance point and half the arrow length, divided by total arrow length. You also enter point weight so the same call can estimate remaining mass under a two-mass model of a tip plus the rest of the shaft. A non-positive arrow length is rejected because the ratio is undefined when there is no length to divide by.
What front of center measures and why archers quote it as a percentage
Front of center is not a vague feel of tip heavy. It is a ratio you can recompute with a tape and a knife-edge stand. Measure total arrow length from the throat of the nock groove to the end of the shaft; Easton and the Archery Manufacturers Organization treat that span as L and they do not include the protruding point in L even though the point is installed when you find the balance.
Half of L is the geometric center of the shaft. Mass in the point, insert, and outsert pulls the balance forward of mid-shaft; fletching, wraps, and a heavy nock pull it back. FOC percent is one hundred times the balance point minus L over two, all divided by L.
A twelve percent FOC means the center of mass sits twelve percent of the shaft length in front of the midpoint. Two arrows with the same FOC can still fly differently if spine, length, and fletching differ, but FOC is the number shops write on a build card because it is reproducible. This calculator implements that identity exactly.
It does not estimate dynamic FOC in flight, does not scrape a spine chart, and does not need GPI when you already measured the balance. Treat the percentage as a static build figure, not as a guarantee of grouping or penetration, and keep length and balance in the same unit so the ratio stays honest.
Practical tips, FOC bands, and when to use this calculator
Do not mix a millimetre length with an inch balance. Target archers often live near seven to twelve percent FOC; many hunting builds sit near twelve to fifteen percent; extreme-FOC hunting setups push fifteen to nineteen percent and beyond. The band is guidance, not a rule: a stiff indoor target arrow and a heavy-bone hunting arrow can share a number and still need different points.
Stay inside the published limits of ten thousand on length and one hundred thousand on point weight, which covers any real shaft and any pile of brass you might screw on. Prefer this capability when you already have the tape measurements and only need the AMO identity. Round-trip checks are easy: thirty and eighteen should return ten percent, and a rest exactly at mid-shaft should return zero.
What you can do with it
Hunting versus target FOC checks
Coaches and hunters compare a target arrow near ten percent FOC with a hunting arrow near twelve to fifteen percent without a spreadsheet formula drifting between notebooks.
Class answer keys and tuning logs
Archery classes and club logs store length, balance, and point mass per arrow, then call this tool once to fill an FOC column that matches the AMO identity byte for byte.
FAQ
How is arrow front of center calculated?
The balance point is measured from the nock-groove throat to the rest, with the point installed; length is nock throat to the end of the shaft, in the same unit.
Why do you ask for point weight if FOC uses length and balance?
The measured rest already includes the installed point, so FOC% does not change with a second copy of the grain figure. Point weight is used to estimate remaining mass under a two-mass model when FOC is not zero, and it is echoed on the build record.
What FOC percentage should I aim for?
Many target builds sit near seven to twelve percent. Hunting arrows often sit near twelve to fifteen percent, and extreme-FOC hunting setups go higher. The calculator labels a band; it does not replace spine, broadhead, and fletching tests.
Is the arrow FOC calculator free?
The browser tool is free and runs locally.
How do I measure arrow length and balance point?
Measure from the throat of the nock groove to the end of the shaft, leaving out the protruding point. Then rest the finished arrow on a narrow edge and measure from the same nock throat to the balance point, in the same unit.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/sports/foc \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"arrow_length":30,"balance_point":18,"point_weight":125}'const res = await fetch("https://api.kit.forhosting.com/sports/foc", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"arrow_length": 30,
"balance_point": 18,
"point_weight": 125
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/sports/foc",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"arrow_length": 30,
"balance_point": 18,
"point_weight": 125
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/sports/foc", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"arrow_length":30,"balance_point":18,"point_weight":125}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"arrow_length":30,"balance_point":18,"point_weight":125}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/sports/foc", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"arrow_length": 30,
"balance_point": 18,
"point_weight": 125
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "sports.foc",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_arrow_length | 10000 |
max_point_weight | 100000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What front of center measures and why archers quote it as a percentage
Rest the finished arrow on a narrow edge and record the distance from that same nock throat to the rest; that distance is the balance_point.
How to call the API, which fields it needs, and how errors are raised
A classic check is length thirty, balance eighteen, point one hundred twenty-five: FOC equals ten percent. Wire the free browser form for shop notes, then call the identical contract from scripts when you log a batch of builds. Send a JSON body with three required numeric fields. The field arrow_length is total arrow length L from the nock-groove throat to the end of the shaft, a positive finite number from a thousandth through ten thousand in whatever length unit you used on the tape, with aliases length, total_length, total_arrow_length, and l. The field balance_point is the rest distance from that same nock throat, a finite number on the closed interval from zero through arrow_length, with aliases balance, bp, balance_point_from_nock, and a. The field point_weight is tip mass, a non-negative finite number typically in grains, with aliases point, tip_weight, grains, and point_grains. When arrow_length is not positive the task fails with invalid_input, because FOC is a ratio and division by a non-positive length is undefined. A balance point past the shaft end, or a negative point weight, fails with invalid_input as well, naming the field that went out of range. On success the body returns foc_percent as the primary percentage, foc as the same ratio on a decimal scale, the echoed inputs, half_length, front_of_center_distance, a band string, the formula, method amo_easton_foc, a short message, and an ordered steps array. When FOC is not zero, implied_remaining_mass and implied_total_mass appear from a two-mass model of a tip at the shaft end plus remaining mass at mid-shaft; those keys are omitted, never null, when the model is not determined. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Practical tips, FOC bands, and when to use this calculator
Copy arrow_length, balance_point, and point_weight from the same finished arrow: the point must be installed when you find the rest, and L must be the nock-to-shaft-end span, not the overall length including the tip. This tool labels those bands as low, target, hunting, high, and extreme, plus tail_heavy when the rest sits behind mid-shaft. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when you deliberately send a zero length to test client validation. This page targets people searching for an arrow FOC calculator, a front of center calculator, or FOC percent from length and balance; it is not a spine selector, not a kinetic-energy tool, and not a broadhead flight simulator. API pricing remains $0.002 per successful request with unit item.
Integration use cases
Shop build cards and point swaps
A technician tapes nock-to-shaft length, finds the rest with the point installed, and records FOC% next to point weight so a later grain change has a baseline that matches the free browser widget.
Technical questions
What happens if arrow length is zero or negative?
The API returns invalid_input. FOC is undefined when total arrow length is not positive, so a zero or negative arrow_length is rejected rather than returning infinity or a fake zero.