Tolerances & Fits Guide
A complete interactive guide to tolerances and fits: IT precision grades, the ISO 286 fit system (H7/f6 and more), ISO 2768 general tolerances, tolerance stack-up analysis (Worst Case vs. RSS), ANSI/ASME inch fits, and the relationship between tolerance and manufacturing cost.
📏 IT Grade — What is a Precision Grade?
ISO 286 defines 20 precision grades (IT01, IT0, IT1...IT18) — each grade roughly doubles the tolerance width of the previous one. The grade does not depend on the direction of deviation (+/-), only on the total width of the allowed range.
Approximate formula (ISO 286): i = 0.45×∛D + 0.001×D [μm], where D = the geometric mean diameter of the size range (mm). IT_n = i × a table coefficient based on n.
📐 IT Grades — Actual Tolerance Values (μm)
| IT | Usage | 3–6mm | 6–10mm | 18–30mm | 50–80mm | 80–120mm |
|---|---|---|---|---|---|---|
| IT01–IT1 | Gauge blocks, lab instruments | 0.6–1 | 0.6–1 | 0.8–1.2 | 1–1.5 | 1.2–1.5 |
| IT2–IT3 | Gauge Blocks, calibration | 1.5–2.5 | 1.5–2.5 | 2–3.5 | 2.5–4.5 | 3–5 |
| IT4 | High-precision bearings, spindles | 4 | 4 | 6 | 10 | 12 |
| IT5 | Precision gears, lapping | 5 | 6 | 9 | 13 | 15 |
| IT6 | Engine shaft, H7/f6 fit, grinding | 8 | 9 | 13 | 19 | 22 |
| IT7 ★ | Representative fits — the most common | 12 | 15 | 21 | 30 | 35 |
| IT8 | General fits, finish milling | 18 | 22 | 33 | 46 | 54 |
| IT9 | Valves, general equipment | 30 | 36 | 52 | 74 | 87 |
| IT10 | Rough production, ordinary turning | 48 | 58 | 84 | 120 | 140 |
| IT11 | Precision castings, rough cutting | 75 | 90 | 130 | 190 | 220 |
| IT12–IT13 | Sand casting, forging | 120–190 | 150–220 | 210–330 | 300–460 | 350–540 |
| IT14 | Sheet cutting, plate | 300 | 360 | 520 | 740 | 870 |
| IT15–IT18 | Rough sand casting, commercial tolerance | 480–1400 | 580–1500 | 840–2100 | 1200–2900 | 1400–3500 |
Rule of thumb for cost: each step down in grade (e.g. IT8→IT7) ≈ 1.5-2× more expensive to machine. Don't choose a tighter grade than functionally required!
⚙️ Typical Precision Grade by Manufacturing Process
Rough Turning / Milling
IT11–IT13
Ra 3.2–6.3μm
No further finishing
Ra 3.2–6.3μm
No further finishing
Standard Turning / Milling
IT9–IT10
Ra 1.6–3.2μm
The default for CNC
Ra 1.6–3.2μm
The default for CNC
Finish Turning / Milling
IT7–IT8
Ra 0.8–1.6μm
Slow feed + sharp tool
Ra 0.8–1.6μm
Slow feed + sharp tool
Grinding
IT5–IT6
Ra 0.2–0.8μm
After hardening
Ra 0.2–0.8μm
After hardening
Honing / Superfinish
IT3–IT5
Ra 0.05–0.2μm
Bearings, pistons
Ra 0.05–0.2μm
Bearings, pistons
Lapping
IT1–IT3
Ra 0.01–0.05μm
Gauge blocks, optics
Ra 0.01–0.05μm
Gauge blocks, optics
🔤 ISO 286 System — Principles
Every fit is defined by a letter (the position of the tolerance zone relative to the nominal size) + a number (the IT Grade — the zone width). For example H7: H=position, 7=width.
Uppercase letters (A...ZC) = holes. Lowercase letters (a...zc) = shafts.
The letter order represents a shift of the tolerance zone: A/a (farthest, huge clearance) → ... → H/h (exactly on the nominal line) → ... → ZC/zc (farthest in the opposite direction, huge interference).
Uppercase letters (A...ZC) = holes. Lowercase letters (a...zc) = shafts.
The letter order represents a shift of the tolerance zone: A/a (farthest, huge clearance) → ... → H/h (exactly on the nominal line) → ... → ZC/zc (farthest in the opposite direction, huge interference).
🏠 Hole-Basis vs. Shaft-Basis System
★ Hole Basis
The hole is always H (lower deviation = 0, fixed minimum size). The fit is set by choosing the shaft letter: H7/f6, H7/k6, H7/p6...
Why it's most common: holes are made with standard tools (drill, reamer, broach) at a fixed diameter. It's easier to change the shaft dimension in turning than to order a special reamer for every fit.
Why it's most common: holes are made with standard tools (drill, reamer, broach) at a fixed diameter. It's easier to change the shaft dimension in turning than to order a special reamer for every fit.
Shaft Basis
The shaft is always h (upper deviation = 0). The housing/hole varies: C11/h9, D9/h9, K7/h6...
When used: when the shaft is an off-the-shelf item (a silver-steel rod h6, a standard shaft) and the housing is made around it. Common in pneumatics, systems with long standard shafts.
When used: when the shaft is an off-the-shelf item (a silver-steel rod h6, a standard shaft) and the housing is made around it. Common in pneumatics, systems with long standard shafts.
🔡 Deviation Letter Table — by Family
| Shaft Letter | Hole Letter | Family | Character |
|---|---|---|---|
| a, b, c | A, B, C | Huge clearance | Large slack, usually not engineering |
| d, e | D, E | Generous clearance | Lightly-loaded plain bearings, free mechanisms |
| f, g | F, G | Small clearance | Free, precise sliding |
| h | H | Zero (on the line) | The system basis — h/H |
| js, k, m, n | JS, K, M, N | Transition | Possible tiny clearance or slight interference |
| p, r, s, t, u | P, R, S, T, U | Interference (press) | Guaranteed interference — press/shrink fit |
| v...zc | V...ZC | Heavy interference | Permanent fixed joints |
The full designation: [letter][number]. ⌀50H7 = nominal 50mm hole, letter H, grade IT7 → +0.030/0mm
🔗 The Three Fit Families
Clearance
Guaranteed clearance in every combination. Free hand assembly.
Use: a rotating shaft in a bushing, a free mechanism
Use: a rotating shaft in a bushing, a free mechanism
Transition
Possible tiny clearance or slight interference, depending on actual dimensions.
Use: precise location with the option to disassemble — gears, pulleys
Use: precise location with the option to disassemble — gears, pulleys
Interference
Guaranteed interference in every combination. Press/heat for assembly.
Use: bearings in a housing, permanent bushings
Use: bearings in a housing, permanent bushings
🔗 H7 Fits — Actual Tolerance Values at ⌀50mm
| Fit | Type | Hole (μm) | Shaft (μm) | Min Clearance | Max Clearance | Assembly | Use |
|---|---|---|---|---|---|---|---|
| H7/d9 | Wide clearance | +25/0 | -80/-130 | +80 | +155 | By hand | Fast motion |
| H7/f6 | Clearance | +25/0 | -25/-50 | +25 | +75 | By hand | Shaft in a bushing |
| H7/g6 | Close clearance | +25/0 | -9/-25 | +9 | +50 | By hand, carefully | Adjustment |
| H7/h6 | Sliding | +25/0 | 0/-16 | 0 | +41 | By hand, precisely | Replaceable parts |
| H7/js6 | Transition | +25/0 | ±8 | -8 | +33 | Mallet/hand | Light location |
| H7/k6 | Transition | +25/0 | +15/+2 | -15 | +23 | Rubber mallet | Pulley, sprocket |
| H7/m6 | Firm transition | +25/0 | +21/+8 | -21 | +17 | Light press | Fixed supports |
| H7/p6 | Interference | +25/0 | +42/+26 | -42 | -1 | Press/heat | Bearings, bushings |
| H7/r6 | Medium interference | +25/0 | +51/+34 | -51 | -9 | Heat | Permanent joints |
| H7/s6 | Heavy interference | +25/0 | +59/+43 | -59 | -18 | Heat only | Gear crowns |
Positive value = clearance · negative value = interference · ⌀50 IT7=30μm, IT6=19μm
🔗 Additional Fits — H8/H9/H11 (wider tolerance)
| Fit | Type | Typical Use |
|---|---|---|
| H8/e8 | Wide clearance | Lightly-loaded plain bearings, heat |
| H8/f7 | Clearance | General mechanisms, free rotation |
| H9/h9 | Sliding — commercial tolerance | General equipment, less critical |
| H11/c11 | Huge clearance | Sheet metal parts, rough assemblies, long shafts |
| H11/h11 | Rough sliding | Machine screws, large acceptable slack |
The higher the IT grade (H8→H9→H11) — the wider and cheaper the tolerance to manufacture. Choose based on actual functional need, not "as precise as possible".
📊 ISO 2768 — General Tolerances for Machining (Part 1: Dimensions)
| Linear Dimensions (mm) | ||||
|---|---|---|---|---|
| Grade | 0.5–3 | 3–30 | 30–120 | 120–400 |
| f (fine) | ±0.05 | ±0.1 | ±0.15 | ±0.2 |
| m (medium) ★ | ±0.1 | ±0.2 | ±0.3 | ±0.5 |
| c (coarse) | ±0.2 | ±0.5 | ±0.8 | ±1.2 |
| v (very coarse) | — | ±1.0 | ±1.5 | ±2.5 |
| Angles (°) | ||
|---|---|---|
| Grade | up to 10mm | 10–50mm |
| f | ±1° | ±0°30' |
| m ★ | ±1° | ±0°30' |
| c | ±1°30' | ±1° |
In the title block: "ISO 2768-mK" = medium (m) linear + K geometric. This designation saves marking a tolerance on every dimension on the drawing!
📐 ISO 2768 Part 2 — General Geometric Tolerances (H/K/L)
| Characteristic | H (fine) | K (medium) ★ | L (coarse) | Size Range |
|---|---|---|---|---|
| Flatness / Straightness | 0.02–0.1 | 0.05–0.4 | 0.1–0.8 | up to 10 to 3000mm |
| Perpendicularity | 0.2–0.5 | 0.4–1 | 0.6–1.5 | up to 100 to 3000mm |
| Symmetry | 0.5 | 0.6–0.8 | 0.6–1.2 | up to 100 to 3000mm |
| Runout | 0.1 | 0.2 | 0.5 | constant for all diameters |
Applies automatically to any GD&T characteristic not explicitly marked on the drawing. K is the common industry default — pairs with ISO 2768-mK.
⚖️ ISO 2768 vs. ASME Y14.5 — an Important Distinction
ISO 2768 is a blanket default tolerance (applies to dimensions with no explicit marking). ASME Y14.5 is the full GD&T standard (Datum, FCF, Position, etc.) — they are not substitutes for each other!
A complete, professional drawing includes both: "ISO 2768-mK" in the title block for "unmarked" dimensions, and explicit FCFs (⊕⌀0.2 A|B|C) on critical characteristics requiring tighter control.
A complete, professional drawing includes both: "ISO 2768-mK" in the title block for "unmarked" dimensions, and explicit FCFs (⊕⌀0.2 A|B|C) on critical characteristics requiring tighter control.
⛓️ Tolerance Stack-up — Why It's Critical
When several parts stack up in an assembly, their tolerances accumulate. Even if each part is individually within its own tolerance — the complete assembly may still fail to close! Two main calculation methods:
Worst Case (WC)
Formula: Gap = Σnominal ± Σtolerances (direct summation)
Assumes every part in the chain is simultaneously at the edge of its tolerance. Completely safe but expensive — requires tightening tolerances on every link to meet the requirement.
When to use: low-volume assemblies, safety-critical applications (aerospace, medical), insufficient parts for statistical analysis.
Assumes every part in the chain is simultaneously at the edge of its tolerance. Completely safe but expensive — requires tightening tolerances on every link to meet the requirement.
When to use: low-volume assemblies, safety-critical applications (aerospace, medical), insufficient parts for statistical analysis.
RSS — Root Sum Square (statistical)
Formula: Gap_tol = √(t1² + t2² + ... + tn²)
Assumes the actual size distribution is random (Normal Distribution) — it's unlikely all parts land at the edge simultaneously. Gives a wider assembly tolerance at the same confidence level.
Requirement: a controlled, centered manufacturing process (Cpk≥1.33), a large enough quantity for a true distribution.
Assumes the actual size distribution is random (Normal Distribution) — it's unlikely all parts land at the edge simultaneously. Gives a wider assembly tolerance at the same confidence level.
Requirement: a controlled, centered manufacturing process (Cpk≥1.33), a large enough quantity for a true distribution.
🔢 Numerical Example — 3-Part Stack
| Part | Nominal Size | Tolerance |
|---|---|---|
| Housing | 50.0mm | ±0.10mm |
| Spacer | 20.0mm | ±0.05mm |
| Shaft | 29.5mm | ±0.08mm |
Nominal: 50 − 20 − 29.5 = 0.5mm final gap
Worst Case: ±(0.10+0.05+0.08) = ±0.23mm → final range: 0.27 to 0.73mm
RSS: ±√(0.10²+0.05²+0.08²) = ±√(0.01+0.0025+0.0064) = ±0.137mm → final range: 0.363 to 0.637mm
Worst Case: ±(0.10+0.05+0.08) = ±0.23mm → final range: 0.27 to 0.73mm
RSS: ±√(0.10²+0.05²+0.08²) = ±√(0.01+0.0025+0.0064) = ±0.137mm → final range: 0.363 to 0.637mm
RSS gives a narrower (more realistic) range — a difference of 0.093mm on each side! In a stack-up with many links, the difference is far more dramatic.
🛠️ Solutions When a Stack-up Doesn't Close
1. Widen tolerances — the cheapest option, if functionally possible
2. Switch to RSS — if the process is statistically controlled (requires real Cpk data!)
3. Add a shim/adjustable spacer — assembly-time adjustment instead of precision manufacturing
4. Redesign — fewer links in the chain = less accumulation (a direct Datum instead of a chain)
5. Selective Assembly — sort parts into groups and match them (costly in labor, cheap in material)
2. Switch to RSS — if the process is statistically controlled (requires real Cpk data!)
3. Add a shim/adjustable spacer — assembly-time adjustment instead of precision manufacturing
4. Redesign — fewer links in the chain = less accumulation (a direct Datum instead of a chain)
5. Selective Assembly — sort parts into groups and match them (costly in labor, cheap in material)
🇺🇸 ANSI/ASME B4.1 — Inch Fit System
The American (pre-ISO) system is still common on older American drawings, standards, and in the aerospace industry. Divided into 8 classes by fit type — completely different from ISO's letters!
| Class | Full Name | Type | Approximate ISO Equivalent |
|---|---|---|---|
| RC | Running / Sliding Clearance | Running clearance | H7/f6...H11/c11 |
| LC | Locational Clearance | Locational clearance | H7/h6, H7/g6 |
| LT | Locational Transition | Locational transition | H7/k6, H7/js6 |
| LN | Locational Interference | Light locational interference | H7/n6, H7/p6 |
| FN | Force / Shrink Fit | Force / shrink | H7/s6, H7/u6 |
📋 RC Table — Common Running/Sliding Fits (⌀2" = 50.8mm)
| Class | Character | Use |
|---|---|---|
| RC1 | Very precise | Instrumentation, measuring tools |
| RC3 | Precise | Shaft in a precision bearing, spindle |
| RC5–RC6 | General ★ | Plain bearings, general industrial equipment |
| RC8–RC9 | Wide | Agricultural equipment, large acceptable slack |
📋 FN Table — Force/Shrink Fits
| Class | Press Force | Assembly | Use |
|---|---|---|---|
| FN1 | Light | Hand press | Light bushings, brass/bronze |
| FN2 | Medium | Hydraulic press | Bearings, industrial gears |
| FN3 | Heavy | Press + heat | Permanent joints, high load |
| FN4–FN5 | Shrink Fit | Heat only (up to 300°C+) | Crowns, maximum-force joints |
TPI (Threads Per Inch) and inch dimensions use the same philosophy — always check whether the drawing/supplier is American (ASME) or metric (ISO) before converting!
💰 The Relationship Between Tolerance and Manufacturing Cost
Tightening tolerance is not linear in cost — it's exponential. Every step toward higher precision requires an entirely different process, not just "working more carefully".
📈 Cost-Tolerance Curve (relative, not absolute)
| Tolerance (at ⌀50mm) | IT Grade | Process | Relative Cost |
|---|---|---|---|
| ±0.5mm | IT12-13 | Rough casting/forging | ×1 (baseline) |
| ±0.2mm | IT10-11 | Rough turning/milling | ×1.3 |
| ±0.05mm | IT8-9 | Standard turning/milling | ×2 |
| ±0.021mm (IT7) | IT7 | Finish milling/turning, slow feed | ×3.5 |
| ±0.010mm | IT6 | Grinding | ×7 |
| ±0.003mm | IT4-5 | Precision grinding, honing | ×15 |
| ±0.001mm | IT2-3 | Lapping, superfinish | ×35+ |
The numbers are relative and vary by geometry/material/quantity — but the exponential trend is consistent across the entire manufacturing industry.
❌ A Common Mistake
An engineer "adds margin" and tightens tolerance "to be safe" — without checking if it's actually functionally required. Result: double-to-triple the cost with no benefit.
✅ The Right Approach
Choose the widest tolerance that still guarantees function (assembly, load, sealing). Everything else — general ISO 2768. Let Stack-up Analysis show where tightening is actually needed.
💡 Practical Tip
Ask: "what happens if this dimension is off by 0.1mm?" If the answer is "nothing" — the tolerance in the current drawing is too tight and can be relaxed to save cost.
🧮 Fit Limits Calculator
Calculate Hole and Shaft Limits
Enter a nominal size and deviations (μm) to calculate limits and the actual fit
—
⛓️ Stack-up Calculator (up to 5 links)
Enter the ± tolerance for each link (mm) — 0 = unused
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▸ Worst Case = Σ|ti| (direct sum)
▸ RSS = √(Σti²) (root sum of squares — valid only in a statistically controlled process)
▸ RSS = √(Σti²) (root sum of squares — valid only in a statistically controlled process)