Laser spot size calculator
A 632.8 nm helium-neon beam 5 mm wide, focused by a lens with a 0.1 m focal length, comes to a spot 0.0000161141196782 m across. Enter wavelength, focal length and beam diameter in one unit to size a cutting, marking or fibre-coupling spot. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Wavelength
- 0.0000006328
- Focal length
- 0.1
- Beam diameter
- 0.005
- Spot diameter
- 0.0000161141
- Spot radius
- 0.0000080571
A collimated laser does not stay infinitely thin when a focusing lens brings it to a waist. Diffraction and the finite diameter of the incoming Gaussian beam set a smallest 1/e² core in the focal plane. For a TEM00 beam focused by a thin lens, the textbook focused laser spot diameter is d equals four times wavelength times focal length, divided by pi times the input beam diameter at the lens.
How a focusing lens sets the TEM00 laser spot diameter
A TEM00 laser is a Gaussian beam: its transverse intensity falls as the square of a Gaussian, and the 1/e² diameter is the conventional width used in laser catalogs and focusing estimates. When that collimated beam of diameter D fills a thin lens of focal length f, the focused 1/e² diameter at the waist is given by the closed-form relation d equals four lambda f over pi D.
The factor four over pi is not an empirical fudge; it follows from the Gaussian-beam ABCD transformation of a collimated waist through a thin lens in the paraxial, low-divergence limit with beam-quality factor M-squared equal to one. Longer wavelength, longer focal length, or a smaller input beam all enlarge the focused core; a shorter wavelength, a tighter lens, or a larger collimated diameter all shrink it. This capability evaluates only that focused TEM00 diameter and half of it as the radius from wavelength, focal length, and input beam diameter.
It does not apply the circular-aperture Airy factor 2.44 lambda N, does not fold in M-squared greater than one, does not compute Rayleigh range or depth of focus, and does not convert focal length and diameter into an f-number for you. Those are different search intents and different landings. Wavelength, focal length, and beam diameter must share one length unit; the returned spot lengths use that same unit.
Where a focused laser spot size calculator earns its keep
Laser process engineers size cutting, welding, marking, and micromachining spots before they pick a focusing optic and a beam expander, and the TEM00 formula from wavelength, focal length, and input beam diameter is the first-order waist they can claim with M-squared equal to one. Fiber-coupling and free-space alignment work compares that focused diameter to a mode-field diameter so the coupling efficiency estimate starts from a honest core size rather than a diffraction slogan.
University optics and photonics courses assign endless problems that ask for the focused 1/e² diameter from lambda, f, and D; a single trusted oracle that returns both diameter and radius speeds grading and demo setup. If you later need the circular-aperture Airy first-null diameter from wavelength and f-number, an f-number from focal length and aperture, numerical aperture conversions, or thin-lens image distance, treat those as separate search intents and landings rather than overloading this laser spot size page.
What you can do with it
Laser cutting, welding and micromachining
Compute the focused 1/e² TEM00 waist from laser wavelength, focusing focal length and collimated beam diameter before sizing optics and claiming a process spot.
Homework and lab answer keys
Return a deterministic focused laser spot diameter and radius from textbook λ, f and D so teachers and auto-graders share one ground truth.
FAQ
What formula does this laser spot size calculator use?
Spot lengths use the same unit as the three inputs. Beam-quality factor M² is taken as one.
Is this the same as the Airy diffraction-limited spot?
No. The circular-aperture Airy first-null diameter is 2.44·λ·N from wavelength and f-number. This tool reports the TEM00 Gaussian focused 1/e² diameter from wavelength, focal length and input beam diameter, which is the usual laser-focusing estimate.
How can I make the focused spot smaller?
Use a shorter wavelength, a lens with a shorter focal length, or a wider beam at the lens, for example with a beam expander. A longer wavelength, a longer focal length or a narrower beam all make the spot bigger.
Does it account for beam quality above the ideal?
No. It assumes an ideal single-mode Gaussian beam with a beam-quality factor of one. A real multimode laser focuses to a larger spot, so treat the result as the smallest waist you can expect.
Which units should I use?
Any single length unit for all three values, such as metres for the wavelength, the focal length and the beam diameter. The spot diameter and radius come back in that 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/phys/laser-spot-size \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"wavelength":6.328e-7,"focal_length":0.1,"beam_diameter":0.005}'const res = await fetch("https://api.kit.forhosting.com/phys/laser-spot-size", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"wavelength": 6.328e-7,
"focal_length": 0.1,
"beam_diameter": 0.005
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/phys/laser-spot-size",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"wavelength": 6.328e-7,
"focal_length": 0.1,
"beam_diameter": 0.005
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/laser-spot-size", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"wavelength":6.328e-7,"focal_length":0.1,"beam_diameter":0.005}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"wavelength":6.328e-7,"focal_length":0.1,"beam_diameter":0.005}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/laser-spot-size", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"wavelength": 6.328e-7,
"focal_length": 0.1,
"beam_diameter": 0.005
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "phys.laser_spot_size",
"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
This laser spot size calculator accepts those three lengths, returns the focused diameter and the matching radius, and fails with invalid_input when the beam diameter is zero instead of emitting Infinity.
How a focusing lens sets the TEM00 laser spot diameter
The pipeline is pure floating-point arithmetic with no network, no random seeds, and no wall-clock timestamps, so identical lambda, f, and D always produce identical JSON. Beam diameter must be strictly positive: a zero input diameter would divide by zero and is rejected as invalid_input rather than Infinity.
What you send and what you get back
All three fields are required. Wire the free browser form for homework and lab demos, then call the identical contract from scripts when you need bulk process tables. Send three finite positive numbers under the fields wavelength, focal_length, and beam_diameter. wavelength is lambda in any length unit you prefer for the answer; focal_length is the focusing-lens focal length f in that same unit; beam_diameter is the incoming 1/e² diameter D at the lens, also in that unit. Aliases such as lambda, wave_length, or wl for the wavelength, f, focal, or fl for the focal length, and D, diameter, input_beam_diameter, beam_d, or beam_dia for the beam diameter are accepted so scripts written against common textbook notation do not need a rename layer. Missing fields, non-numeric strings, NaN, Infinity, zero or negative wavelength, zero or negative focal length, and zero or negative beam diameter fail with invalid_input and are not charged. On success the response echoes cleaned wavelength, focal_length, and beam_diameter, returns spot_diameter equal to four lambda f over pi D, returns spot_radius equal to half of that diameter, includes a short formula string d equals 4 lambda f over pi D, and a unit_note reminding you that spot lengths match the three input lengths. Classic checks include a helium-neon wavelength six hundred thirty two point eight nanometres written as six point three two eight times ten to the minus seven metres, a one hundred millimetre lens written as zero point one metres, and a five millimetre input beam written as zero point zero zero five metres, giving a tens-of-micrometres focused core; a neodymium YAG wavelength one point zero six four micrometres at a fifty millimetre lens and a ten millimetre beam; and a zero beam diameter as the mandatory invalid_input path. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where a focused laser spot size calculator earns its keep
Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Integration use cases
Fiber coupling and focusing-optic design
Compare d = 4λf/(πD) to a fiber mode-field diameter or a target feature size when choosing a focusing lens and input beam expansion.
Technical questions
What units should I use?
Any consistent length unit for wavelength, focal_length and beam_diameter (metres, millimetres, micrometres). The returned spot_diameter and spot_radius are in that same unit. Example: λ = 6.328e-7 m, f = 0.1 m, D = 0.005 m yields d in metres.
What happens if the beam diameter is zero?
The request fails with invalid_input. A zero (or non-positive) beam diameter would divide by zero in d = 4λf/(πD). Wavelength and focal_length must also be finite and strictly positive; validation failures are not charged.
What does the laser spot size calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.