Wood beam span calculator
A Douglas Fir-Larch No.2 two-by-ten at 16 inches on center under 40 psf spans a little over 16 feet, limited by deflection. Use it for early layout, then confirm with your local span tables. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Species grade
- douglas_fir_larch_no2
- Beam size
- 2x10
- Spacing
- 16
- Load
- 40
- Species grade label
- Douglas Fir-Larch No.2 ft
- Spacing ft
- 1.333333
- Width in
- 1.5
- Depth in
- 9.25
Choosing a wood beam is not just picking a stamped size off a rack. The maximum distance that member can run between supports shrinks when the on-center spacing widens, because a wider tributary strip of floor or roof load becomes a heavier line load on the beam, and it also shrinks when the species grade is weaker or the uniform load is higher. This wood beam span calculator accepts a species and grade, a nominal beam size, the on-center spacing in inches, and a uniform load in pounds per square foot, then returns a preliminary allowable span in feet governed by the tightest of bending, shear, and L/360 deflection. Two-by stock spaced twenty-four inches on center or closer receives the repetitive-member factor used in common framing; four-by and six-by sections are treated as single beams. Use the result for early layout and material checks. Confirm final sizes against the span tables adopted by your jurisdiction and a licensed professional before you cut posts, set hangers, or pour footings.
How species grade, size, spacing, and load set the allowable span
A wood beam in this model is a simply supported rectangular member that carries a uniform strip of floor or roof. The strip width is the on-center spacing converted from inches to feet, so a sixteen-inch spacing is one and one-third feet of tributary width and a ninety-six-inch spacing is an eight-foot bay between parallel beams. Uniform load in pounds per square foot times that tributary width is the line load in pounds per foot.
Nominal sizes such as two-by-ten and four-by-twelve use standard surfaced dimensions: a two-by-ten is one and a half by nine and a quarter inches, a six-by-twelve is five and a half by eleven and a half. Section modulus S equals b d squared over six and moment of inertia I equals b d cubed over twelve. Species grade maps to approximate single-member reference values for bending stress Fb, modulus of elasticity E, and horizontal shear Fv.
A simplified size factor raises Fb for shallower depths, and two-by members at twenty-four inches on center or closer also receive a repetitive-member factor of one point one five. Bending capacity limits span through the classic w L squared over eight moment formula. Shear uses the parabolic average for a rectangular section.
Deflection enforces five w L to the fourth over three hundred eighty-four E I no greater than L over three hundred sixty, using the same uniform load.
What you can do with it
Floor and loft layout
Enter species grade, beam size, on-center spacing, and uniform load to estimate how far a wood beam or joist can run between supports before the preliminary allowable span is exceeded.
Compare grades and sections
Switch from No.2 to Select Structural, or from a two-by-ten to a four-by-ten, at the same spacing and load to see which change restores enough span for the planned bay.
FAQ
What units are spacing and load?
spacing is on-center spacing in inches (twelve, sixteen, nineteen-point-two, and twenty-four are typical for joists; enter ninety-six for an eight-foot beam bay). load is uniform design load in pounds per square foot. Both must be finite positive numbers.
What happens if load or spacing is not a positive number?
Zero, negative, missing, and non-finite values for load or spacing are rejected and are not billed.
Which species grades and beam sizes are supported?
Twelve species-grade tokens covering Douglas Fir-Larch, Southern Pine, Hem-Fir, and Spruce-Pine-Fir in Select Structural, No.1, and No.2. Sizes run 2x6–2x12, 4x6–4x12, and 6x6–6x12 using standard dressed dimensions.
What limits the span: bending, shear or deflection?
Whichever is tightest. The calculator checks bending strength, shear and a deflection limit of span over 360, reports the shortest span and names the check that governed, so you know whether to add depth for stiffness or width for strength.
Can I build directly from this span?
No. It is a preliminary estimate from simple-span beam theory and approximate species values. Confirm the final size against the span tables your jurisdiction adopted and a licensed professional before cutting posts or pouring footings.
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/build/wood-beam-span \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"species_grade":"douglas_fir_larch_no2","beam_size":"2x10","spacing":16,"load":40}'const res = await fetch("https://api.kit.forhosting.com/build/wood-beam-span", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"species_grade": "douglas_fir_larch_no2",
"beam_size": "2x10",
"spacing": 16,
"load": 40
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/build/wood-beam-span",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"species_grade": "douglas_fir_larch_no2",
"beam_size": "2x10",
"spacing": 16,
"load": 40
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/build/wood-beam-span", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"species_grade":"douglas_fir_larch_no2","beam_size":"2x10","spacing":16,"load":40}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"species_grade":"douglas_fir_larch_no2","beam_size":"2x10","spacing":16,"load":40}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/build/wood-beam-span", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"species_grade": "douglas_fir_larch_no2",
"beam_size": "2x10",
"spacing": 16,
"load": 40
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "build.wood_beam_span",
"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.
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
How species grade, size, spacing, and load set the allowable span
The reported span in feet is the minimum of those three checks, and governing_limit names which check controlled the answer so you can see whether you need more depth for stiffness or more width for strength.
Input fields, response shape, and what happens when load or spacing is invalid
Invalid species-grade or beam-size tokens fail the same way. Example: Douglas Fir-Larch No.2, a two-by-ten, sixteen inches on center, and forty pounds per square foot returns a finite span in feet limited by the governing check for that configuration. Keep spacing in inches and load in psf so the line load is correctly pounds per linear foot. Required fields are species_grade, beam_size, spacing, and load. species_grade is one of twelve combinations: Douglas Fir-Larch, Southern Pine, Hem-Fir, or Spruce-Pine-Fir, each in Select Structural, No.1, or No.2. beam_size is a nominal size from two-by-six through two-by-twelve, four-by-six through four-by-twelve, and six-by-six through six-by-twelve. spacing is on-center spacing in inches and must be a finite positive number no greater than two hundred forty. load is uniform design load in pounds per square foot and must be a finite positive number no greater than five hundred. Numeric fields accept JSON numbers or numeric strings. Aliases grade and species, size and section, spacing_in and oc, and load_psf and psf are accepted. If load or spacing is missing, zero, negative, non-finite, or otherwise not a positive number, the task fails with invalid_input naming the field and is not billed. On success the payload echoes the resolved grade and size, reports width_in and depth_in, section_modulus_in3, moment_of_inertia_in4, adjusted Fb_prime_psi, the repetitive factor, line load in pounds per foot, separate span_bending_ft, span_shear_ft, and span_deflection_ft candidates, then the primary span_ft and span_in with governing_limit.
When a wood beam span estimate helps and what it does not replace
Homeowners sketching a floor or loft need a fast way to see whether a two-by-ten at sixteen inches on center will clear a planned bay under a forty-pound live-style load, without opening a full engineering workbook. Framing crews and remodelers use the same estimate when a client changes joist direction, beam spacing, or species mid-bid and someone has to re-check post or bearing-wall layout before lumber is ordered. Always compare the output to the wood beam and joist span tables adopted by your jurisdiction and obtain professional review for load-bearing residential floors, roofs, headers, and any condition outside simple single-span gravity loading. Software that generates material takeoffs can call the API with species_grade, beam_size, spacing, and load to flag spans that exceed a conservative preliminary limit and push the user toward a deeper section, a tighter spacing, or an extra support. Every successful API call costs the published $0.002 base price with no model tokens and no outbound network; validation failures do not consume balance. This calculator is intentionally a preliminary tool: it does not encode every species, wet-service factor, notch reduction, cantilever, continuous multi-span redistribution, snow-drift combination, or repetitive-member exception that a stamped design may require.
Integration use cases
Bid and takeoff sanity checks
Call the API from estimating tools when spacing, load, or species grade changes mid-quote so weak beam layouts are flagged before lumber is ordered.
Technical questions
How does the wood beam span calculator work?
It builds a uniform line load from load (psf) times spacing converted to feet, sizes the rectangular section from beam_size, applies approximate species-grade Fb, E, and Fv with size and repetitive factors, and takes the minimum span from bending, shear, and L/360 deflection for a simple span. The result is span_ft.
What does the wood beam span calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.