IT Grade Tolerance Calculator
IT Grade Tolerance Calculator
The ISO 286 standard tolerance for any grade from IT01 to IT18 at any size from 1 to 500 mm, derived from the standard’s own tolerance factor i = 0.45·∛D + 0.001·D and its grade multipliers — with the manufacturing processes that hold each grade, and the reverse: the finest grade that covers a tolerance you already have.
IT grade tolerance
IT7 on a Ø25 mm feature
One factor, and a multiplier per grade
- D
- the GEOMETRIC mean of the size step’s two limits, not the nominal size itself. For the 18 to 30 mm step that is √540 = 23.24 mm, and every diameter in the step uses it
- i
- the standard tolerance factor in micrometres. The cube-root term is the machining part — error grows with the cube root of the size — and the linear term is the measuring part, because a longer part is harder to measure
- IT(n)
- the standard tolerance, the WIDTH of the band. It says nothing about where the band sits; that is the letter’s job
- 1.585
- 10 to the one-fifth, the ratio between consecutive grades. It is why five grades is a factor of ten and why the ladder feels evenly spaced on a logarithmic scale
Worked example
IT7 on a Ø25 mm feature
Find the size step and its geometric mean. 25 mm is in “over 18 to 30”, so D = √(18 × 30) = 23.238 mm. Not 25, and not 24 — the standard uses the geometric mean of the step, which is why every size from 18.001 to 30.000 gets the same answer
The standard tolerance factor: i = 0.45·∛23.24 + 0.001 × 23.24 = 0.45 × 2.8536 + 0.0232 = 1.3074 µm
IT7 is 16i, so the formula gives 16 × 1.3074 = 20.918 µm. The standard prints 21 µm, which is 0.39% higher — it rounded up to a convenient number. That rounding is not itself a formula, which is why this page carries the table and uses the formula as the check rather than the other way round
So an IT7 hole at Ø25 is 21 µm wide: 25.000 to 25.021 if it is an H7, 24.979 to 25.000 if it is an h7, ±10.5 µm if it is a JS7. The grade fixes the WIDTH and the letter fixes the position — two separate decisions that get confused constantly
Now the consequence that decides the cost. A turning operation holds IT7 to IT11, and grinding holds IT5 to IT8. IT7 is at the fine end of turning and the coarse end of grinding, so it is the last grade you can ask for without committing the part to a grinder. Ask for IT6 — 13 µm, only 38% narrower — and on most shops' process sheets you have specified a grinding operation, a second setup and a second inspection
And the size dependence, which surprises people. The same 21 µm band is IT7 at Ø25 but IT5 at Ø200 (where IT5 is 20 and IT6 is 29). Tolerance grows as the cube root of size, so holding an absolute band on a big part is much harder than on a small one. A 50 µm band, for contrast, is IT9 at Ø30 and IT7 at Ø200
Finally, the number that tells you whether you need a room rather than a machine. A steel part changes size by α·D·ΔT = 11.7 × 10⁻⁶ × 25 × ΔT, so the whole IT7 band here is eaten by a temperature change of 72 K. That is comfortable. At IT4 it would be 21 K, and at IT1 about 5.1 K — which is why fine grades are a metrology-laboratory subject and not a machining one
Which process holds which grade, and what that is in micrometres at Ø25 mm
| Process | IT grades it holds | Finest band at Ø25 mm (µm) | Coarsest band at Ø25 mm (µm) | Note |
|---|---|---|---|---|
| Lapping | IT01 to IT5 | 0.6 | 9.0 | The finest size control there is, and the only family that reaches IT01. |
| Honing | IT4 to IT7 | 6.0 | 21.0 | Bore size and finish together; the usual finishing operation on a hydraulic cylinder. |
| Cylindrical grinding | IT5 to IT8 | 9.0 | 33.0 | The workhorse for a shaft that has to fit something. |
| Surface grinding | IT5 to IT8 | 9.0 | 33.0 | Flat features to the same grades. |
| Diamond boring | IT5 to IT7 | 9.0 | 21.0 | A single-point finishing cut, used where a grinding wheel cannot reach. |
| Broaching | IT5 to IT7 | 9.0 | 21.0 | Holds size on an internal form in one stroke. |
| Reaming | IT6 to IT10 | 13.0 | 84.0 | A hole to size after drilling; the cheapest way to an IT7 bore. |
| Turning | IT7 to IT11 | 21.0 | 130.0 | IT7 routinely; IT6 only with a sharp tool, a light finishing pass and a temperature-stable setup. |
| Boring | IT7 to IT11 | 21.0 | 130.0 | The same grades as turning, from the inside. |
| Milling | IT8 to IT11 | 33.0 | 130.0 | One grade coarser than turning for the same care. |
| Planing and shaping | IT10 to IT11 | 84.0 | 130.0 | Long flat surfaces, coarse. |
| Drilling | IT10 to IT13 | 84.0 | 330.0 | A drilled hole is not a sized hole. |
| Cold rolling and extruding | IT10 to IT11 | 84.0 | 130.0 | |
| Stamping | IT10 to IT14 | 84.0 | 520.0 | |
| Die casting | IT10 to IT13 | 84.0 | 330.0 | Better than sand casting by three grades. |
| Powder metallurgy, formed | IT5 to IT7 | 9.0 | 21.0 | Tighter than most people expect. |
| Powder metallurgy, sintered | IT6 to IT9 | 13.0 | 52.0 | |
| Sand casting and flame cutting | IT14 to IT18 | 520.0 | 3,300.0 | Machining allowance, not a fit. |
| Forging | IT14 to IT16 | 520.0 | 1,300.0 |
Every IT grade at seven sizes, in micrometres
| Grade | Ø3 | Ø10 | Ø25 | Ø50 | Ø100 | Ø200 | Ø400 | Multiple of i |
|---|---|---|---|---|---|---|---|---|
| IT01 | 0.3 | 0.4 | 0.6 | 0.6 | 1.0 | 2.0 | 3.0 | tabulated |
| IT0 | 0.5 | 0.6 | 1.0 | 1.0 | 1.5 | 3.0 | 5.0 | tabulated |
| IT1 | 0.8 | 1.0 | 1.5 | 1.5 | 2.5 | 4.5 | 7.0 | tabulated |
| IT2 | 1.2 | 1.5 | 2.5 | 2.5 | 4.0 | 7.0 | 9.0 | tabulated |
| IT3 | 2.0 | 2.5 | 4.0 | 4.0 | 6.0 | 10.0 | 13.0 | tabulated |
| IT4 | 3.0 | 4.0 | 6.0 | 7.0 | 10.0 | 14.0 | 18.0 | tabulated |
| IT5 | 4.0 | 6.0 | 9.0 | 11.0 | 15.0 | 20.0 | 25.0 | 7 i |
| IT6 | 6.0 | 9.0 | 13.0 | 16.0 | 22.0 | 29.0 | 36.0 | 10 i |
| IT7 | 10.0 | 15.0 | 21.0 | 25.0 | 35.0 | 46.0 | 57.0 | 16 i |
| IT8 | 14.0 | 22.0 | 33.0 | 39.0 | 54.0 | 72.0 | 89.0 | 25 i |
| IT9 | 25.0 | 36.0 | 52.0 | 62.0 | 87.0 | 115.0 | 140.0 | 40 i |
| IT10 | 40.0 | 58.0 | 84.0 | 100.0 | 140.0 | 185.0 | 230.0 | 64 i |
| IT11 | 60.0 | 90.0 | 130.0 | 160.0 | 220.0 | 290.0 | 360.0 | 100 i |
| IT12 | 100.0 | 150.0 | 210.0 | 250.0 | 350.0 | 460.0 | 570.0 | 160 i |
| IT13 | 140.0 | 220.0 | 330.0 | 390.0 | 540.0 | 720.0 | 890.0 | 250 i |
| IT14 | 250.0 | 360.0 | 520.0 | 620.0 | 870.0 | 1,150.0 | 1,400.0 | 400 i |
| IT15 | 400.0 | 580.0 | 840.0 | 1,000.0 | 1,400.0 | 1,850.0 | 2,300.0 | 640 i |
| IT16 | 600.0 | 900.0 | 1,300.0 | 1,600.0 | 2,200.0 | 2,900.0 | 3,600.0 | 1000 i |
| IT17 | 1,000.0 | 1,500.0 | 2,100.0 | 2,500.0 | 3,500.0 | 4,600.0 | 5,700.0 | 1600 i |
| IT18 | 1,400.0 | 2,200.0 | 3,300.0 | 3,900.0 | 5,400.0 | 7,200.0 | 8,900.0 | 2500 i |
What each part of the ladder is actually for
| Grades | Where they live | What it costs to ask for them |
|---|---|---|
| IT01 to IT4 | Gauge blocks, master rings, plug gauges, measuring instrument spindles. Not part features | A temperature-controlled room and a lapping or honing operation. At Ø25 an IT4 band is 6 µm, and a steel part of that size changes by 6 µm over a 20 K excursion — so the tolerance and the room temperature are the same problem |
| IT5 to IT7 | The fit grades. Bearing seats, spindle journals, gear bores, dowel holes, anything whose clearance matters | Grinding, honing, reaming, fine boring. This is where the preferred fits live and where the money goes on a precision part |
| IT8 to IT11 | General machining. Clearance holes, bosses, spigots, features that locate loosely or not at all | Turning, milling, drilling and reaming with ordinary care. IT11 is where the loose-running fits sit |
| IT12 to IT14 | Stamped, cast and moulded features, and the default for an untoleranced dimension on many drawings | Nothing extra. ISO 2768’s medium class is roughly IT13 to IT14 at most sizes, which is why a general tolerance note covers these grades without saying so |
| IT15 to IT18 | Sand castings, forgings, flame-cut plate, machining allowances | Nothing, and they are not fits. An IT16 band at Ø200 is 2.5 mm |
One factor, a multiplier per grade, and the process that the grade quietly specifies
An IT grade is a width, not a position, and it is computed rather than looked up. ISO 286 defines a standard tolerance factor i = 0.45·∛D + 0.001·D micrometres, where D is the GEOMETRIC mean of the nominal size step — √(18 × 30) = 23.24 mm for the 18 to 30 mm step, not 25 and not 24. Every grade from IT5 up is a fixed multiple of it: IT5 = 7i, IT6 = 10i, IT7 = 16i, IT8 = 25i, IT9 = 40i, IT10 = 64i, IT11 = 100i and so on. The ratio between consecutive grades is about 1.585, which is ten to the one-fifth, so five grades is exactly a factor of ten — and that rule is exact from IT7 upward, at all sixty-five cells checked here, which is where this page’s IT17 and IT18 come from rather than from a table.
The formula gets you to about a part in thirty, and the page says so. Compared with the published table, i and the grade multipliers reproduce every cell from 6 mm up to within 5.3 per cent and 146 of 154 to within 3 per cent; the 3 to 6 mm step to 8.5 per cent; and the 1 to 3 mm step only to 13.3 per cent, always rounded UP. The reason is that ISO 286’s values were rounded to convenient numbers by hand when the system was written, and that rounding is not recoverable as a rule. So this page carries the table and uses the formula as the check — and reports the gap, because anyone deriving ISO 286 from its formulas alone deserves to know the size of the error they are accepting. IT01 to IT4 are tabulated for the same reason, though IT2, IT3 and IT4 satisfy the standard’s other construction rule — a geometric progression from IT1 to IT5 — at twelve of the thirteen steps.
Which process holds which grade is the practical content, and the consequence is a cost. Lapping reaches IT01 to IT5, honing IT4 to IT7, cylindrical and surface grinding IT5 to IT8, reaming IT6 to IT10, turning and boring IT7 to IT11, milling IT8 to IT11, drilling IT10 to IT13, die casting IT10 to IT13, and sand casting IT14 and coarser. Read that list backwards and the important line appears: asking for IT6 on a turned feature specifies a grinding operation whether the drawing says so or not, and that is a second setup, a second machine and a second inspection for a band about 38 per cent narrower. A second published source puts fine turning at IT6 to IT7 with reduced feed and a temperature-stable setup, and both readings are in the table, because the disagreement is exactly the answer to whether IT6 is a turned grade: obtainable, not routine.
Tolerance grows with size, so the same band is a different grade at a different diameter. A 50 µm band is IT9 at Ø30 and IT7 at Ø200 — two grades apart for the same absolute number. A 21 µm band is IT7 at Ø30 and IT5 at Ø200. That is the cube-root term at work, and it has a practical consequence people get wrong in both directions: a large part at a given grade is proportionally far MORE accurate than a small one (an IT7 band is 840 parts per million of the size at Ø25 and 230 at Ø200), while holding a fixed absolute band on a large part is much harder than on a small one. If you are specifying by absolute micrometres out of habit, the series on this page is worth a minute.
And there is a temperature underneath all of it. A steel part changes size by about 11.7 parts per million per kelvin, so at Ø25 the whole IT7 band is eaten by a 72 K excursion, the IT4 band by 21 K and the IT1 band by about 5 K. That is why fine grades are a metrology-laboratory subject rather than a machining one, and why ISO 1 fixes 20 °C as the reference temperature for every dimensional specification. The thermal effect on fit calculator does that correction, and the go / no-go gauge tolerance calculator does the other half of the same squeeze — how much of the band the gauges themselves take.
Frequently asked questions
What is IT7 at 25 mm?
21 µm. The 18 to 30 mm step has a geometric mean of 23.24 mm, so i = 0.45·∛23.24 + 0.001 × 23.24 = 1.307 µm, and IT7 is 16i = 20.9 — which ISO 286 rounds up to 21. An H7 hole at Ø25 is therefore 25.000 to 25.021 mm, an h7 shaft is 24.979 to 25.000, and a JS7 feature is ±10.5 µm.
Which IT grade can I hold on a lathe?
IT7 to IT11 routinely, for turning and boring alike. IT6 is the boundary and the interesting case: the process grid this page uses stops turning at IT7, so on most shops’ process sheets IT6 means grinding — but a second published source says fine turning reaches IT6 to IT7 with reduced feed, sharp tooling and a temperature-stable setup. Both are in the table. The safe reading is that IT6 is obtainable on a good lathe and is not a grade to specify casually; IT7 is one step away and 38 per cent wider at Ø25.
Why does the same tolerance correspond to different grades at different sizes?
Because manufacturing error grows with size, and ISO 286 builds that in through the cube-root term in i. A 50 µm band is IT9 at Ø30 and IT7 at Ø200; a 21 µm band is IT7 at Ø30 and IT5 at Ø200. Two consequences worth holding on to. Specifying an absolute tolerance across a range of sizes silently makes the large parts easy and the small parts hard. And a big part at a given grade is proportionally much more accurate than a small one — IT7 is 840 parts per million at Ø25 and 230 at Ø200.
What is the difference between IT7 and H7?
IT7 is a WIDTH and H7 is a width and a position. IT7 at Ø25 says the band is 21 µm wide and says nothing about where it sits; H7 says the band is 21 µm wide AND that its lower limit is exactly the nominal size, so the hole is 25.000 to 25.021. The letter is the lever that changes a fit; the grade only changes how much the fit varies. People reach for the grade when the clearance is wrong, and it is almost always the letter that needs changing.
Where do IT17 and IT18 come from?
From the standard’s own rule rather than from a table. ISO 286 makes IT(n+5) exactly ten times IT(n) from IT7 upward, and this page verified that against fetched IT12 to IT16 rows at all sixty-five cells before using it — the agreement is exact, not approximate. So IT17 is ten times IT12 and IT18 is ten times IT13. Note that the same rule does NOT hold lower down: IT10 is not ten times IT5 and IT11 is not ten times IT6, so it cannot be extended downward.
How tight can a tolerance be before temperature matters?
Sooner than people expect. A steel part moves about 11.7 parts per million per kelvin, so at Ø25 an IT7 band of 21 µm is eaten by a 72 K excursion, an IT4 band by 21 K and an IT1 band by about 5 K. The results panel gives that number for whatever grade and size you enter. When it drops below about five kelvin the drawing has to state the reference temperature — ISO 1 makes 20 °C the default — and the measurement has to be corrected or the room controlled.
Related calculators
References
- ISO 286-1:2010, Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits. Cited by number; the standard is copyrighted and its tables are not reproduced here. What this page uses from it is its STRUCTURE, which is not a table: the standard tolerance factor i = 0.45·∛D + 0.001·D evaluated at the geometric mean of each nominal size step, the grade multipliers (IT5 = 7i, IT6 = 10i, IT7 = 16i, IT8 = 25i, IT9 = 40i, IT10 = 64i, IT11 = 100i and so on), the rule that IT(n+5) is ten times IT(n) from IT7 upward, the formulas for each letter’s fundamental deviation, and the Δ correction for hole letters J to ZC. Every one of those is checked here against a published table rather than trusted.
- Firat Bearing. ISO 286 — IT tolerance grades reference. The source for the IT01 to IT4 and IT12 to IT16 rows. The IT12 to IT16 rows turned out to be EXACTLY ten times IT7 to IT11 at all sixty-five cells, which is ISO 286-1’s own rule, so IT17 and IT18 are derived from it here rather than copied. The IT2, IT3 and IT4 rows satisfy the standard’s other construction rule — a geometric progression from IT1 to IT5 — at twelve of the thirteen size steps.
- Justway. Machining tolerance guide: precision tolerance grade, including a “Summary Table of IT Grades for Various Machining Methods”. The source for the process-capability grid on this page: which IT grades each of twenty processes can hold. Its IT6 row also agrees with two independent IT tables at all thirteen size steps, which is why it is trusted for the grid.
- UTEC Industry Resource Center. ISO tolerance grades (IT grades) explained for CNC machining. The second, independently written process-capability source, in prose rather than a grid. It agrees with the grid on grinding, honing, reaming and drilling and disagrees at two edges — it puts routine CNC turning at IT7–IT9 against the grid’s IT7–IT11, and says fine turning reaches IT6–IT7 “with reduced feed, sharp tooling and temperature-stable setup” where the grid stops turning at IT7. Both readings are printed, because the disagreement is the answer to whether IT6 is a turned grade.
- Machining Doctor. Tolerance charts by fundamental deviation letter (one page per letter, hole and shaft, stated as ISO 286-1:2010). The source for the fundamental deviations of c, d, e, f, g, k, m, n, p, r, s and u on this page, fetched one letter at a time so a shifted column could not hide in a wide table. Every row of every letter satisfies es − ei = IT(grade) exactly — 220 independent checks — and its N7 hole column reproduces the Δ = IT(n) − IT(n−1) rule at all thirteen size steps. Its u6 column also agrees with the interference-fit page already on this site at all sixteen steps that page covers, which cross-validates both.
- ISO 1:2022, Geometrical product specifications (GPS) — Standard reference temperature for the specification of geometrical and dimensional properties. Its freely published preview was fetched during this batch. Clause 4: “The standard reference temperature value for the specification of geometrical and dimensional properties shall be fixed at tₐ₀ = 20 °C.” Annex A adds the part that matters for measurement: most measuring systems “are designed to produce measurement results specified at the standard reference temperature value of 20 °C, even though they might be operating at a different temperature”, and a departure from it is additional measurement uncertainty.