ISO 2768 General Tolerances: What mK vs fH Costs in a Quote
September 28, 2026
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"General tolerances ISO 2768-mK" is the most-ignored line on the title block. It sits in small print under the material callout, it applies to every dimension on the drawing that carries no individual tolerance — which on a typical job-shop part is most of them — and almost nobody reads it before pricing the job. Then a drawing arrives with ISO 2768-fH in the same small print, the estimator quotes it like the last one, and the shop discovers at inspection that a 350 mm fabricated part now owes the customer ±0.2 mm instead of ±0.5 mm on every untoleranced length. Understanding what ISO 2768 general tolerances actually demand, class by class, is one of the cheapest pieces of estimating knowledge there is: it costs a table and ten minutes, and it protects every quote you send.
This guide covers the standard from the estimator's side: what the two parts and their classes mean, the actual tolerance values by dimension range, when a general-tolerance note is free because your process already holds it, when it silently raises cost — fine class on large dimensions, formed and welded parts, flexible sections — and the checklist to run on every title block before the price goes out. The companion pieces on the tolerance-to-cost relationship and GD&T for estimators cover individually toleranced features; this one covers everything those callouts leave behind.
What ISO 2768 actually says
ISO 2768 has two parts, and a title-block note like "2768-mK" quotes them in order. Part 1 covers linear and angular dimensions without individual tolerances, in four classes: f (fine), m (medium), c (coarse), and v (very coarse). Part 2 covers geometrical tolerances — straightness, flatness, perpendicularity, symmetry, run-out — for features without individual callouts, in three classes: H (tightest), K, and L (loosest). So mK means medium dimensional tolerances plus class-K geometry, the most common combination on machined-part drawings; fH means fine dimensions plus the tightest general geometry, and it is a meaningfully harder drawing wearing the same font size.
Two things estimators routinely get wrong about the standard. First, it applies only to dimensions without individual tolerances — a dimension with its own ±0.05 keeps its ±0.05 regardless of class. Second, the tolerance is not one number but a table: it widens with the dimension. Class m allows ±0.1 on a 5 mm dimension and ±0.8 on an 800 mm one. That scaling is exactly why the standard is usually livable — and why the fine class stops being livable at the large end, as we will see.
The numbers: ISO 2768-1 linear tolerances by class
This is the table to pin above the estimating desk. Permissible deviations in mm for linear dimensions, by nominal dimension range:
| Nominal dimension (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
|---|---|---|---|---|
| 0.5 up to 3 | ±0.05 | ±0.1 | ±0.2 | — |
| over 3 to 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| over 6 to 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| over 30 to 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| over 120 to 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| over 400 to 1000 | ±0.3 | ±0.8 | ±2.0 | ±4.0 |
| over 1000 to 2000 | ±0.5 | ±1.2 | ±3.0 | ±6.0 |
Read it like an estimator, not like an inspector. In the small ranges, f and m are one tick apart — ±0.05 against ±0.1 — and any machining center holds either without noticing. In the large ranges the classes diverge hard: at 800 mm, m allows ±0.8 while f allows ±0.3, and on a welded or formed part that difference is the difference between "as it comes" and "fixture, check, and rework". Part 2 behaves the same way: general flatness under class K is 0.2 mm on a 100 mm face but the class-H value halves it, and perpendicularity under K starts at 0.4 mm for legs up to 100 mm. The class letter is constant across the drawing; its cost is not — it lives at the big dimensions.
When general tolerances are free
Here is the good news that keeps quoting honest: on the right process, most general-tolerance classes cost nothing, because the process capability is already inside the table. A CNC machining center in decent condition holds ±0.05 as routine work; every f and m value from the table above, up through the mid ranges, falls out of a normal toolpath with no extra passes, no extra inspection, no operator attention. For machined parts up to a few hundred millimetres, quoting 2768-f versus 2768-m should change nothing in your price, and an estimator who adds a "fine class" premium to a small milled bracket is inventing cost.
The discipline is to know, per process, where your capability sits against the table. As 2026 calibration points: CNC milling and turning comfortably hold class f through the 120 mm range and usually beyond; laser cutting holds m on profile dimensions all day and f with attention; press-brake forming holds m on single bends but starts fighting f as soon as dimensions cross bends; manual TIG-welded fabrications hold c naturally and m with fixturing. Where the drawing's class sits at or below your process line, the note is free — price the part as if it were not there. Where it sits above, it is a work instruction, and work instructions cost money.
When the class bites: large dimensions, bends, and welds
The fine class earns its price premium in three places. Large dimensions first: ±0.2 over 120–400 mm is easy on a machining center and genuinely demanding on anything formed, welded, or flame-cut, where thermal distortion and springback move material by more than that on their own. A 2768-f note on a fabricated frame quietly converts a weldment into a straighten-measure-rework cycle. Note also that welded constructions have their own general-tolerance standard, ISO 13920, with far looser values — a customer who puts 2768-f on a weldment has often simply used the wrong template, and a quote-stage question can save both sides real money, the same way catching drawing ambiguities early reduces quoting errors.
Dimension chains across bends second: a flange-to-flange dimension over two press-brake bends stacks two springback variations. Class m absorbs that stack; class f frequently does not, which means a first-off check against the table, possibly a re-strike, and per-part verification — real minutes on the press-brake cost calculation that a flat "per bend" rate never included. Thin and flexible parts third: the part 2 geometry classes assume the part holds its own shape. A 2 mm sheet panel 600 mm long can flex more than its class-K flatness allowance under its own weight; measuring it at all requires agreeing on how it is supported, and that conversation belongs at quote time, not at the inspection bench.
The cost delta, worked in euros
Take a laser-cut, press-brake-formed bracket, S235, 350 mm overall with two bends and a hole pattern, batch of 50. Under 2768-mK the governing untoleranced dimensions carry ±0.5 (over the 120–400 range) and the brake holds that with a standard setup: no extra first-off iterations, spot-check inspection, negligible scrap. Under 2768-fH the same dimensions owe ±0.2 and class-H geometry:
| Cost item | 2768-mK | 2768-fH |
|---|---|---|
| Brake setup and first-off | €40 | €70 (extra iteration + gauge) |
| In-process checks | spot | 1 min/part → €33 at €40/h |
| Scrap / rework allowance | ~0% | 3% → ~€25 on the batch |
| Delta on a €900 batch | — | ≈ €90, or +10% |
Ten percent is a realistic, defensible premium for fH over mK on formed work of this size — and the point of the exercise is that it is calculable, not a vibe. On a purely machined part of the same size the same table would read close to zero delta, because the mill holds both classes. The class note costs whatever the gap between the table and your process capability costs, and that gap you can price line by line: extra setup iterations, added inspection minutes, and a scrap allowance where capability is marginal.
The estimator's title-block checklist
Five checks, thirty seconds, on every incoming drawing. First, find the note — no 2768 reference and no individual tolerances means tolerances are contractually undefined, and your quote should state what you will work to (a sentence that has settled many disputes). Second, read both letters: the dimensional class and the geometry class move cost independently, and an mH or fK hybrid deserves a closer look than the usual mK. Third, scan for the largest untoleranced dimension and look it up in the table — that cell, not the class letter, is the real requirement. Fourth, match the class against each process in the routing: a part that is machined, formed, and welded must satisfy the class at every stage, and the weakest process sets your cost. Fifth, check for the wrong standard on the wrong part — 2768-f on weldments and large fabrications is usually a template accident worth a clarifying question rather than a silent premium or, worse, a silent acceptance.
Run the checklist and the small print stops being a trap. Skip it, and every fH drawing you price as mK is margin you donated before the job ever hit the floor.
FAQ: ISO 2768 and quoting
What does ISO 2768-mK mean? It combines the two parts of the standard: "m" is the medium class for linear and angular dimensions without individual tolerances (for example ±0.3 on dimensions from 30 to 120 mm, ±0.5 from 120 to 400 mm), and "K" is the middle class of general geometrical tolerances for flatness, perpendicularity, and symmetry from part 2. It is the most common general-tolerance note on machined-part drawings, and most CNC processes hold it without extra cost.
Does the ISO 2768 class change the price of a part? It depends on the gap between the class and your process capability. On small and mid-size machined parts, f and m are both inside normal CNC capability, so the class changes nothing. On large dimensions, formed parts, and weldments, the fine class demands tighter results than the process delivers naturally, and the price rises through extra setup, per-part inspection, and scrap allowance — often 5–15% on formed work.
How much more does 2768-f cost than 2768-m? As 2026 calibration: near zero on machined parts up to a few hundred millimetres, because milling and turning hold both classes routinely. On press-brake and welded work, expect roughly 5–15% on the affected operations, driven by first-off iterations, added inspection minutes, and scrap where capability is marginal — around 10% on a typical formed bracket batch, as the worked example above shows.
What tolerance applies if the drawing specifies nothing at all? Formally, none — a drawing with no individual tolerances and no general-tolerance note has contractually undefined limits, which protects nobody. Good practice is to state in the quote what you will work to (for example "unless otherwise specified, ISO 2768-mK applies") so the assumption is on paper. Many customers' templates default to 2768-mK precisely to close that hole.
Does ISO 2768 apply to welded assemblies? Welded constructions have their own general-tolerance standard, ISO 13920, with classes suited to thermal distortion — its allowances are much wider than 2768's. A 2768-f or even 2768-m note on a large weldment is often a title-block template accident; query it at quote stage, because pricing a weldment to machining-grade general tolerances makes the quote uncompetitive or the job unprofitable.
Reading the small print at quoting speed
The title-block note is exactly the kind of input that gets lost when quotes are built under pressure: it is small, it is boilerplate on ninety drawings out of a hundred, and on the hundredth it changes the price by ten percent. A human estimator scanning for material and the big dimensions will miss the day the class quietly switched from mK to fH — not from carelessness, but because boilerplate trains people to stop reading it.
QuoteBuddy reads the whole drawing, title block included. It surfaces the general-tolerance note alongside material, dimensions, and finish callouts, so the class is a visible input to the estimate instead of small print nobody re-checked — and the deterministic cost engine applies your own rules for what each class costs on each process, the same way on every quote. The AI does the reading; your calibration decides whether fH on formed work carries the 10% it deserves.
Start a 30-day trial and run your last few drawings through it — including the one with the tolerance note you never read. See what the title block has been quietly asking for, and what your quotes should have been charging for it.