Engineering Fits & Tolerances – Calculator & Charts

Tolerance & Fit Calculator

Engineering Tolerances

The most commonly used tolerancing system for shafts and holes is detailed in ISO 286-1 & 286-2. The first provides the charts for the fundamental deviations (G, j, etc.) and tolerance grades (7, 8, 9, etc.), out of which the limits of the tolerance classes (H7, g6, etc.) can be calculated. ISO 286-2 is a collection of dozens of charts that tabulate the size limits for all the tolerance grades. The figures listed in ISO 286-2 are calculated based on ISO 286-1 and can also be obtained independently. A thorough understanding of the basic terminology is critical for a solid understanding of the data provided by our calculator and charts.

Pay attention that some of the terms have a somewhat different meaning within the tolerancing system (Compared to its general meaning in mechanical engineering!)

Terminology

Basic Size (Also may be called nominal size)

Nominal size is the dimension by which a feature is identified for convenience. It is also the point from which the limits are derived by application of the necessary deviation and tolerance (See explanation below). For example, a slot whose actual width is 25.15 mm would be known as a 25-mm wide slot.

Limits of Size

Limits are the basic size’s high (Upper limit) and low (Lower limit) values. For example, if the lower limit of a hole is 25.05 mm and the upper limit of the same hole is 25.15 mm, then a hole that is 25.1 mm in diameter is within limits and is acceptable. The limits are the final result obtained from the tolerance system based on the nominal size and tolerance class.

Shaft

Shaft refers to an external feature of a workpiece, typically cylindrical in shape. However, the term can be used to describe any external feature. For example, a block that has to fit into a keyway or the square shank of a cutting tool can also be referred to as a shaft.

Hole

Hole refers to an internal feature of a workpiece, typically a bore. However, the term can be used to describe any internal feature. For example, a keyway.

Fundamental Deviation

• Fundamental Deviation is The position of the tolerance zone in relation to the zero line (Also referred to as basic size or nominal size).
•  The fundamental deviation is measured to the point that is nearest to the zero line.
•  A 1-2 character letter code (e.g. G, js, are used to indicate it. Holes are marked by capital letters, and shafts by small letters. For example, “G” is a fundamental deviation of a hole, and “js” is a fundamental deviation of a shaft.
• The size of the deviation depends on its letter code and the nominal size.
• Values are listed in the below table.
• Check out our below infographic for easier understanding.

Upper & lower Deviations

The algebraic difference between the upper limit and the basic size is called the upper deviation and is denoted by es for shafts and ES for holes. The algebraic difference between the lower limit and the basic size is called the lower deviation and is represented by ei for shafts and EI for holes. The configurations may be confusing and depend if the feature is external or internal and if the deviation is negative, positive, or zero.

Deviation Symbol
A-G
H	es=0,ei=IT	EI=0, ES=IT
JS	ei=es=IT/2	EI=ES=IT/2
K-Z

Tolerance

• Tolerance is the difference between the limits of size. That is the upper limit minus the lower limit.
• Standard Tolerance Grade: A number between 0 to 18 and designated as “IT” (e.g. IT7). A lower IT means higher accuracy. The value of each IT also depends on the basic size. You can find the values in the below chart.
• The IS0 system provides 20 standard tolerance grades, of which IT1 to IT18 are considered general use. In addition, there are two complementary grades, IT01 and IT0. The collection of charts provided in ISO 286-2 covers only IT1-IT18.
• Tolerance Class: The term used for the combination of a fundamental deviation with a standard tolerance grade, e.g. h9, D13, etc.

All the above terms are summarized in the below infographics

(*) Click to enlarge

Nomenclature

Standard Tolerance Grades

The standard tolerance grades are designated by the letters IT followed by a number, for example, IT7. When the tolerance grade is associated with a fundamental deviation symbol to form a tolerance class, the letters IT are omitted (e.g., h7, JS5)

Fundamental Deviation

A 1-2 letter code represents the fundamental deviation. Upper case (e.g., G, JS) designates hole deviations, while lower case (e.g., g, js) designates shaft deviations.

Tolerance Class

The tolerance class is designated by the letter(s) representing the fundamental deviation followed by the standard tolerance grade number. For example, a shaft with a deviation of g and a tolerance grade of T11 will be denoted as g11. A hole with a deviation of JS and a tolerance grade of IT7 will be represented as JS7.

Tolerance Size

The tolerance size combines the basic size with the tolerance class, designated by the basic size, followed
by the designation of the required tolerance class (without spaces). For example, 32H7 specifies a hole with a nominal size of 32 mm, deviation H, and tolerance grade of IT7.

Tolerances Charts

Standard Tolerance Grades (IT) Chart

• Tolerance Dimensions in Microns
• To convert to millimeters, divide by 1000
• To convert to Thous, divide by 25.4

Fundamental deviation of holes Chart

• Click the Tolerance Link (e.g., H ➢) to get the Full & Accurate charts for a specific deviation in Inch and Metric units
• Click  icons to show all columns for the entire diameter range (0-3,150 mm)
• The deviations of J through Z also depend on the IT Therefore, the values in the below table are only accurate in some cases. To get the correct value in all cases, click the Tolerance Link (e.g., H ➢).
• Dimentions in Microns. To convert to millimeters, divide by 1000, to Thous, divide by 25.4

Fundamental deviation of shafts Chart

• Click the Tolerance Link (e.g., g ➢) to get the Full & Accurate charts for a specific deviation in Inch and Metric units
• Click  icons to show all columns of the entire diameter range (0-3,150 mm)
• The deviations of J through Z also depend on the IT Therefore, the values in the below table are only accurate in some cases. To get the correct value in all cases, click the Tolerance Link (e.g., g ➢).
• Dimentions in Microns. To convert to millimeters, divide by 1000, to Thous, divide by 25.4

Engineering Fits

Engineering fit refers to the degree of tightness or looseness between two mating parts in an assembly. It is defined by combining a hole with a tolerance class and a shaft with a tolerance class. (See definition of terms above).
Several standards define recommended pairings for different applications. The values presented in our charts and Calculator are based on ISO 286-1:2010 (In earlier revisions, it was separated into ISO 1829, which is now obsolete). In North America, additional fit combinations are sometimes used per ANSI B4.1 & B4.2. The ANSI tolerancing system is nearly equivalent to ISO 286 but not identical. The difference is typically 0.1-0.5 thous.

Main Fit Types

Clearance Fit

A clearance fit is a type in which the shaft is smaller than the hole it is inserted into, allowing for free movement, assembly, and disassembly. Clearance fits can range from loose-running for assemblies that need a significant gap for free movement or to accommodate temperature variation to locational clearance for stationary parts that need to be freely assembled/disassembled but still maintain accuracy.

Transition Fit

A transition fit is a type in which the shaft may be slightly smaller or larger than the hole (before assembly). It is used to create a tight and secure connection between them in applications requiring high precision and stability. It provides high accuracy, but the connecting parts can be disassembled and reassembled without damage. However, force cannot be transmitted.

Press Fit (Interference)

An interference (Press) fit is a type in which the shaft is larger than the hole (before assembly). It creates a frictional force that holds the parts together without the need for additional fasteners or adhesives. The interference fit is commonly used in mechanical engineering to create solid and reliable connections between components that need to withstand heavy loads or vibrations. However, It is hard to disassemble without damaging the parts. Some degree of force can be transmitted between the components.

Fits are also divided into two types of “basis”

There are no technical differences between the two basis. Therefore the choice of the system should be based on economic reasons.

Hole Basis Fits

The hole deviation is zero (meaning an H), and the shaft deviation varies to create different fit levels. For example, H9/d8 , H7/g6. It is the first choice of combinations for general use, since it avoids an unnecessary multiplicity of tools (e.g., reamers) and gauges

Shaft Basis Fits

The shaft deviation is zero (meaning an h), and the hole deviation varies to create different fit levels. For example, D9/h9, G7/h6. This method should only be utilized when it offers clear economic benefits, such as in situations where it is required to assemble multiple parts with holes that have varying deviations onto a single shaft.

Fits Standard Combinations Charts

For ordinary engineering purposes, only a small number of the many (~150,000) possible fit combinations is required. ISO 286-1:2010 lists about 80 fit combinations that meet most of the needs of any engineering task. You can choose from this list or define a custom combination in our Fit Calculator.

(*) In North America, additional fit combinations are sometimes used per ANSI B4.1 & B4.2. You can find below the ISO charts also the ANSI Charts.

Click the Fit Link (e.g. G7/h6 ➢) to get its detailed tolerances in both Inch and millimeters

Fit Combinations Per ANSI B4.1

The tolerance classes mentioned in ANSI B4.1 (e.g., h9, g6, etc.) have the same nomenclature as in ISO 286. It could lead a confusion since the ANSI standard also defines the fundamental deviations and the tolerance grade on its own. The ANSI tolerancing system is nearly the same as the ISO 286 but not identical. If you take the values from the standard ISO charts or use our Online Calculator, you should expect about 0.1-0.5 thous errors.

Fit Combinations Per ANSI B4.2

The tolerance classes mentioned in ANSI B4.1 (e.g., h9, g6, etc.) have the same nomenclature as in ISO 286. It could lead a confusion since the ANSI standard also defines the fundamental deviations and the tolerance grade on its own. The ANSI tolerancing system is nearly the same as the ISO 286 but not identical. If you take the values from the standard ISO charts or use our Online Calculator, you should expect about 0.1-0.5 thous errors.