ANSI B4.1 Limits & Fits Calculator
Hole and shaft limits, clearance or interference and fit type for the 34 inch classes RC, LC, LT, LN and FN, sizes to 19.69 in
ANSI B4.1 is the US inch counterpart of ISO 286: the same idea of a hole zone and a shaft zone compared on one nominal size, but the tables are written in thousandths of an inch and the fits are named by class rather than by letter and grade. RC1–RC9 are running and sliding fits, LC1–LC11 locational clearance, LT1–LT6 locational transition, LN1–LN3 locational interference and FN1–FN5 force and shrink fits. All 34 classes are hole basis: the hole is always an H zone starting at the nominal size, and the shaft moves.
This calculator looks the class up for you. Enter the nominal size in inches, choose a class, and it returns the hole and shaft limits in thou and as finished diameters to five decimals, the largest and smallest clearance or interference the pair can produce, the fit type and the tolerance-zone diagram. The metric equivalents are shown alongside, and the full table of the chosen class for every size range is printed below the result, so a drawing can be checked without opening the standard.
Each class carries an ABC symbol that maps onto the ISO letters — RC3 is H7/f6, LC2 is H7/h6, LT3 is H7/k6, LN2 is H7/p6, FN2 is H7/s6 — so an ISO fit and its ANSI cousin give near-identical clearances at the same size. They are not interchangeable on a drawing, though: the size ranges differ (13 inch ranges up to 19.69 in against 25 metric ranges to 500 mm), the grade tables are rounded separately, and a few cells of the ANSI tables are adjusted by hand. When the drawing is in inches, use these tables; when it is in millimetres, use the ISO 286 calculator.
Class RC3 (H7/f6) for every size
Limits in thousandths of an inch (0.001 in = 25.4 µm) as published in ANSI B4.1. Hole lower limit is always 0 (hole basis).
| Size range (in) | Hole H7 | Shaft f6 | Limits (thou) |
|---|---|---|---|
| 0 – 0.12 | +0.4 / 0 | -0.3 / -0.55 | 0.3 – 0.95 clearance |
| 0.12 – 0.24 | +0.5 / 0 | -0.4 / -0.7 | 0.4 – 1.2 clearance |
| 0.24 – 0.4 | +0.6 / 0 | -0.5 / -0.9 | 0.5 – 1.5 clearance |
| 0.4 – 0.71 | +0.7 / 0 | -0.6 / -1 | 0.6 – 1.7 clearance |
| 0.71 – 1.19 | +0.8 / 0 | -0.8 / -1.3 | 0.8 – 2.1 clearance |
| 1.19 – 1.97 | +1 / 0 | -1 / -1.6 | 1 – 2.6 clearance |
| 1.97 – 3.15 | +1.2 / 0 | -1.2 / -1.9 | 1.2 – 3.1 clearance |
| 3.15 – 4.73 | +1.4 / 0 | -1.4 / -2.3 | 1.4 – 3.7 clearance |
| 4.73 – 7.09 | +1.6 / 0 | -1.6 / -2.6 | 1.6 – 4.2 clearance |
| 7.09 – 9.85 | +1.8 / 0 | -2 / -3.2 | 2 – 5 clearance |
| 9.85 – 12.41 | +2 / 0 | -2.5 / -3.7 | 2.5 – 5.7 clearance |
| 12.41 – 15.75 | +2.2 / 0 | -3 / -4.4 | 3 – 6.6 clearance |
| 15.75 – 19.69 | +2.5 / 0 | -4 / -5.6 | 4 – 8.1 clearance |
Fit class: RC, LC, LT, LN, FN
RC — Running and sliding fits
Parts that turn or slide in one another. The clearance grows with the size so that the running behaviour, with room for lubricant, stays similar across the range. RC1 is the closest, RC9 the loosest.
- RC1 · H5/g4 — Close sliding: Accurate location of parts that must assemble without perceptible play — the closest running class.
- RC2 · H6/g5 — Sliding: Parts move and turn easily but are not meant to run freely; on larger sizes they can seize with a small temperature change.
- RC3 · H7/f6 — Precision running: The closest fit that still runs freely: precision machinery at slow speeds and light journal pressures, no temperature swings.
- RC4 · H8/f7 — Close running: Accurate machinery at moderate surface speeds and journal pressures where accurate location and minimum play matter.
- RC5 · H8/e7 — Medium running: Higher running speeds or heavy journal pressures, or both.
- RC6 · H9/e8 — Medium running, wider: Like RC5 with a wider hole tolerance (H9) — higher speeds and pressures, cheaper to make.
- RC7 · H9/d8 — Free running: Running fit where accuracy is not essential, or with large temperature variations, or both.
- RC8 · H10/c9 — Loose running: Parts made to wide commercial tolerances, as-rolled or cold-drawn shafting.
- RC9 · H11 — Loose running, widest: The widest running fit of the standard — exposed, dirty or heated assemblies.
LC — Locational clearance fits
Stationary parts that must be assembled and taken apart freely and locate each other. LC1–LC4 have zero minimum clearance (h shafts); LC5–LC11 add a small to large guaranteed clearance.
- LC1 · H6/h5 — Locational, closest: H6/h5, zero minimum clearance — precision location of stationary parts that slide together by hand.
- LC2 · H7/h6 — Locational, snug: H7/h6, zero minimum clearance — the general-purpose snug fit for dowels, gauges, covers and spigots.
- LC3 · H8/h7 — Locational, wider: H8/h7, zero minimum clearance, wider tolerances than LC2.
- LC4 · H10/h9 — Locational, commercial: H10/h9, zero minimum clearance, commercial tolerances.
- LC5 · H7/g6 — Locational, small clearance: H7/g6 — a small guaranteed clearance: assembles by hand and still locates accurately.
- LC6 · H9/f8 — Locational, moderate clearance: H9/f8 — free assembly with moderate clearance.
- LC7 · H10/e9 — Locational, larger clearance: H10/e9 — larger clearance for easy assembly of ordinary parts.
- LC8 · H10/d9 — Locational, large clearance: H10/d9 — large clearance, commercial tolerances.
- LC9 · H11/c10 — Locational, loose: H11/c10 — very loose location.
- LC10 · H12 — Locational, very loose: Grades 12/11 — very wide tolerances, generous clearance.
- LC11 · H13 — Locational, widest: Grades 13/12 — the widest class, for rough parts and castings.
LT — Locational transition fits
Accurate location where a little clearance or a little interference are both acceptable. js shafts (LT1, LT2) are centred on the size, k shafts (LT3, LT4) lean towards interference, n shafts (LT5, LT6) are mostly interference.
- LT1 · H7/js6 — Transition, light (js6): H7/js6 — equal chance of a small clearance or a small interference; assembles by hand.
- LT2 · H8/js7 — Transition, light (js7): H8/js7 — like LT1 with wider tolerances.
- LT3 · H7/k6 — Transition, medium (k6): H7/k6 — leans towards interference; assembled with a light tap. Not defined below 0.24 in.
- LT4 · H8/k7 — Transition, medium (k7): H8/k7 — like LT3 with wider tolerances. Not defined below 0.24 in.
- LT5 · H7/n6 — Transition, heavy (n6): H7/n6 — mostly interference; light press, still dismountable.
- LT6 · H7/n7 — Transition, heavy (n7): H7/n7 — like LT5 with a wider shaft tolerance.
LN — Locational interference fits
Rigid, accurate location of parts that are pressed together but do not transmit load through the fit itself. Light (n5), medium (p6) and heavy (r6) interference.
- LN1 · H6/n5 — Interference, light: H6/n5 — small interference for rigid location of parts that carry no load through the fit.
- LN2 · H7/p6 — Interference, medium: H7/p6 — press fit for bushings, dowels and locating rings.
- LN3 · H7/r6 — Interference, heavy: H7/r6 — the tightest locational fit, press assembled.
FN — Force and shrink fits
Interference fits that hold torque and axial load by friction, with roughly constant bore pressure across the sizes. From light drive (FN1) to the heaviest shrink fit (FN5).
- FN1 · H6 — Light drive: Small interference for thin sections, long fits or cast-iron outer members; light assembly pressure.
- FN2 · H7/s6 — Medium drive: Ordinary steel parts, shrink fits on light sections — the tightest fit usable with high-grade cast iron.
- FN3 · H7/t6 — Heavy drive: Heavier steel parts, shrink fits on medium sections. Not defined below 0.95 in.
- FN4 · H7/u6 — Force: Highly stressed parts, or shrink fits where the pressing forces would be impractical.
- FN5 · H8/x7 — Force, heaviest: The largest interference of the standard — shrink assembly of heavily loaded parts.
Frequently Asked Questions
What is ANSI B4.1?
ANSI B4.1-1967 (R2009), "Preferred Limits and Fits for Cylindrical Parts", is the American inch standard for holes and shafts. It defines five families of hole-basis fits — RC running and sliding, LC locational clearance, LT locational transition, LN locational interference and FN force and shrink — with limits tabulated in thousandths of an inch for nominal sizes up to 19.69 in. Its metric twin is ANSI B4.2, which follows ISO 286.
What does class RC3 mean?
RC3 is a precision running fit: an H7 hole with an f6 shaft. At 1.000 in the hole is 1.0000–1.0008 in and the shaft 0.9987–0.9992 in, so the clearance is between 0.8 and 2.1 thou. It is the closest fit that still runs freely and is meant for precision machinery at slow speeds and light journal pressures where the temperature does not change much.
How do ANSI B4.1 classes compare with ISO 286 fits?
Each class has an ABC symbol built from the same letters and grades as ISO 286: RC3 = H7/f6, RC4 = H8/f7, LC2 = H7/h6, LC5 = H7/g6, LT3 = H7/k6, LT5 = H7/n6, LN2 = H7/p6, FN2 = H7/s6, FN4 = H7/u6. At the same nominal size the clearances agree within a fraction of a thou, but the size ranges and the rounding of the tolerance grades differ, so a drawing in inches should quote the ANSI class and a drawing in millimetres the ISO class.
What is a thou?
One thousandth of an inch: 0.001 in = 0.0254 mm = 25.4 µm. ANSI B4.1 tables are written entirely in thou, so a hole limit of +0.8 means the hole may be up to 0.0008 in larger than the nominal size. The calculator shows every value in thou, in inches to five decimals and in millimetres.
Which class for a press-fit bushing or a running bearing?
A bronze bushing pressed into a housing is typically LN2 (H7/p6) or, for thin walls, FN1; the shaft running inside the bushing is RC3 or RC4 depending on speed and accuracy, RC5–RC6 for higher speeds and heat. Gears and pulleys keyed to a shaft use LT3–LT5; parts assembled by hand that must locate accurately use LC2 or LC5. For rolling bearings follow the bearing catalogue rather than these general classes.