ForHosting KIT · Unit converters

Metric to SAE converter

The closest SAE wrench to 13 mm is 1/2 inch, or 12.7 mm, 0.3 mm under the metric hex. Type the size from a bolt head or drawing and get the nearest sixteenth, its size in mm and whether it runs tight or loose. Free in your browser.

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Worked example

If you enter

Mm: 13

You get

Result: 1/2 in
Inches: 0.5118110236
Sae: 1/2
Sae fraction: 1/2

A metric-to-SAE wrench converter answers the shop-floor question of which fractional-inch socket or combination wrench is closest to a millimetre size stamped on a bolt head or listed on a drawing. Enter a positive finite size in millimetres; the tool divides by the exact international factor of twenty-five point four millimetres per inch, rounds the inch value onto the standard SAE sixteenth-inch wrench grid, and reduces that sixteenth to a mixed or proper fraction such as one-half or one and one-sixteenth. The response also reports exact decimal inches, the chosen SAE size in millimetres, and the millimetre difference so you can judge a tight versus a loose fit before you pick up a tool.

What a metric-to-SAE wrench conversion actually rounds

The millimetre size on a metric bolt, nut, or socket is the across-flats dimension of the hex, not a thread pitch and not a shank length. SAE tools in the United States, Canada, and much of the automotive aftermarket still label that same across-flats dimension as a reduced fraction of an inch—one-quarter, five-sixteenths, three-eighths, seven-sixteenths, one-half, nine-sixteenths, five-eighths, eleven-sixteenths, three-quarters, and so on through the two-inch range.

Because one inch is defined as exactly twenty-five point four millimetres, converting is a single exact division followed by a rounding step onto the SAE wrench grid. That grid, for combination wrenches and sockets, steps in sixteenths of an inch: multiply the exact inch value by sixteen, round to the nearest integer, and interpret the integer as sixteenths. Reducing the fraction then yields the label stamped on the tool.

Thirteen millimetres becomes about zero point five one two inches, which is closer to eight sixteenths (one-half inch, twelve point seven millimetres) than to nine sixteenths. Ten millimetres lands on three-eighths, nineteen millimetres on three-quarters, and twenty-seven millimetres on one and one-sixteenth. The millimetre remainder tells you whether the SAE tool will feel slightly loose or slightly tight on that hex; the converter does not invent a custom size between sixteenths, because no such wrench exists in a standard SAE set.

Zero and negative millimetre values are rejected because they are not wrench or bolt sizes.

Where a metric-to-SAE check saves time

Use this converter when a drawing, bolt head, or imported fastener is metric and the only tools on the bench are SAE fractional wrenches. A mobile mechanic can type 13, 17, or 19 and immediately grab one-half, eleven-sixteenths, or three-quarters without unfolding a paper chart that lives in a drawer. A United States warehouse receiving metric hardware can fill an SAE equivalent column on the packing list so pickers who think in fractions still find a fitting socket on the first try.

Prefer a length converter when you need exact decimal inches without rounding onto a wrench grid, and prefer a thread-pitch converter when the question is millimetres per thread rather than across-flats size. Stay inside the published magnitude limit, send a bare number, and you get a reproducible SAE fraction every time.

What you can do with it

Shop-floor wrench matching

Type the millimetre size on a bolt head and pick the nearest SAE socket or combination wrench without a printed conversion chart.

Receiving metric hardware in SAE shops

Fill an SAE-equivalent column on packing lists so pickers who think in fractions still grab a fitting tool for imported metric fasteners.

FAQ

How do I convert a metric wrench size in mm to SAE?

Send a positive finite millimetre size as mm.

Why does 13 mm become 1/2 inch?

13 ÷ 25.4 is about 0.512 in. The nearest SAE sixteenth is 8/16, which reduces to 1/2 in (12.7 mm).

What happens if mm is zero, negative, or not a number?

A wrench or bolt size must be a positive finite number; those failures are not billed.

Is 25.4 exact, and why round to sixteenths?

One inch is exactly 25.4 millimetres. SAE combination wrenches and sockets step in sixteenths of an inch, so the converter rounds onto that grid instead of emitting an arbitrary 64th that no standard wrench matches.

What does the millimetre size on a bolt measure?

The distance across the flats of the hex head, the part the wrench grips. It is not the thread pitch and not the shank length, so it is the number to convert when you need a matching SAE tool.

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

POSThttps://api.kit.forhosting.com/conv/metric-to-sae

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/conv/metric-to-sae \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"mm":13}'

const res = await fetch("https://api.kit.forhosting.com/conv/metric-to-sae", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "mm": 13
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/conv/metric-to-sae",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "mm": 13
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/metric-to-sae", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"mm":13}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"mm":13}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/metric-to-sae", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "mm": 13
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "conv.metric_to_sae",
  "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

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Limits

max_abs 1000000000000

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

Input that is missing, zero, negative, or not a finite number fails with invalid_input rather than a guessed size.

Input contract, response shape, and error behaviour

Send a JSON body with one required field named mm: a positive finite number of millimetres. Aliases millimeters, millimetres, size, value, and metric are accepted so forms and SDKs can use the name that matches the product. Zero, negative numbers, NaN, infinities, empty strings, and non-numeric text fail with invalid_input naming mm, and those failures are not billed. On success the body echoes rounded mm, reports inches as millimetres divided by twenty-five point four rounded to twelve decimal places, sets sae to the mixed-number label such as 1/2 or 1-1/16, sae_fraction to the reduced improper fraction, numerator and denominator of that improper fraction, whole plus frac_numerator and frac_denominator of the mixed form, sae_inches and sae_mm of the chosen grid point, difference_mm and difference_inches as metric minus SAE so a positive remainder means the metric hex is larger, result as the same SAE label, unit in, from_unit mm, to_unit in, mm_per_inch as the exact constant, sae_step as sixteen, formula describing the sixteenth rounding, message as a short English summary, and steps as ordered substitution lines you can paste into a worksheet. Pair thirteen millimetres with the expected one-half inch result in tests; the twelve-decimal rounding is part of the contract so Worker and browser JSON match byte for byte.

Integration use cases

CAD and BOM dual-dimension hex sizes

Export millimetre across-flats values to reduced SAE inch fractions in build scripts with the same deterministic rounding the widget uses.

Technical questions

What does the metric to SAE converter cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →

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