## What is a bilateral tolerance?

A bilateral tolerance is a plus or minus tolerance (+/-). It allows variation from the nominal size in both a positive and negative direction.

In most cases, the bilateral tolerance will be specified as equal in both directions such as 10.0mm +/- 0.5mm.

This is not always the case though and a bilateral tolerance does not need to have equal positive and negative tolerances. 10.0mm +0.2mm/-0.3mm would be an acceptable bilateral tolerance as well.

Here are some quick bilateral tolerance examples:

• 5.5″ +/- 0.25″
• 5.5″ +1.0″/-0.5″
• 25.5mm +0.1mm/-0.2mm
• 30.6mm +/- 0.3mm

Notice that in each of the examples there is allowed variation (tolerance) from the nominal size in both directions.

A tolerance of 25.5mm +0/-0.2mm would not be a bilateral tolerance because it has no tolerance in the positive direction. This would be an example of a unilateral tolerance.

## Bilateral tolerance symbol

There is no GD&T symbol for a bilateral tolerance.

Per ASME Y14.5, the notation for a bilateral tolerance is to show a plus and a minus tolerance associated with a nominal dimension and neither of them is a zero.

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## How to read a bilateral tolerance

• 5.5″ +/- 0.25″
• 5.5″ +1.0″/-0.5″
• 25.5mm +0.1mm/-0.2mm
• 30.6mm +/- 0.3mm

Now let’s break it down so you can see what the nominal size is as well as the top and bottom ends of the tolerance zone.

 Nominal Size Bottom of Tolerance Top of Tolerance 5.5" 5.25" 5.75" 5.5" 5.0" 6.5" 25.5mm 25.3mm 25.6mm 30.6mm 30.3mm 30.9mm

## Types of bilateral tolerances

### Equal bilateral tolerance

An equal bilateral tolerance will have equal plus and minus tolerances such as 6.35 +/- 0.025 as shown in the example above.

### Unequal bilateral tolerance

An unequal bilateral tolerance will have plus and minus tolerances that are not the same and neither is zero such as 17.0 +0.1/-0.2 as shown in the example above.

## Bilateral tolerances compared to other tolerance types

### Bilateral tolerance vs unilateral tolerance

A bilateral tolerance allows a tolerance in both directions.

A unilateral tolerance allows a tolerance in only one direction.

A bilateral tolerance is plus AND minus tolerance. A unilateral tolerance is a plus OR minus tolerance.

Here are some examples of unilateral tolerances:

• 10.0mm +0/+0.5mm
• 5.515″ +0.010″/+0.015″
• 2.325″ +0/-0.005″
• 4.5mm -0.2/-0.3mm

### Bilateral tolerance vs limit tolerance

A bilateral tolerance specifies a nominal size and a plus/minus tolerance. These values are used to determine the tolerance range for a feature.

A limit tolerance skips the calculation step and simply gives you the tolerance range.

The table below shows bilateral tolerances with their equivalent limit tolerances.

 Bilateral Tolerance Limit Tolerance 10.0 +/-0.5 9.5 - 10.5 5.525 +0.025/-0.050 5.475 - 5.550 7.55 +/+0.15 7.40 - 7.70 2.324 +0.005/-0.010 2.314 - 2.329

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## What is a unilateral tolerance?

Basically, a unilateral tolerance is a type of tolerance that is only allowed in one direction. Either an all plus tolerance or an all minus tolerance.

Here are some quick unilateral tolerance examples:

• 10.0mm +0/+0.5mm
• 5.515″ +0.010″/+0.015″
• 2.325″ +0/-0.005″
• 4.5mm -0.2/-0.3mm

Notice that in these examples, all of the allowed size variation is in one direction. The direction can be positive or negative and zero is allowed.

Unilateral tolerances are often used to specify dimensions that require a specific fit with a mating part.

## Unilateral tolerance symbol

There is no GD&T symbol for a unilateral tolerance.

Per ASME Y14.5, the notation for a unilateral tolerance is to show a plus or a minus tolerance associated with a nominal dimension. It is acceptable for one of the specified tolerances to be zero.

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## How to read a unilateral tolerance

• 10.0mm +0/+0.5mm
• 5.515″ +0.010″/+0.015″
• 2.325″ +0/-0.005″
• 4.5mm -0.2/-0.3mm

Now let’s break it down so you can see what the nominal size is as well as the top and bottom ends of the tolerance zone.

 Nominal Size Bottom of Tolerance Top of Tolerance 10.0mm 10.0mm 10.5mm 5.515" 5.525" 5.530" 2.325" 2.320" 2.325" 4.5mm 4.2mm 4.3mm

Regardless of what the nominal size is, the requirement for each of these unilateral tolerance examples would be that the dimension must fall within the top and bottom tolerance range.

## Unilateral tolerances compared to other tolerance types

### Unilateral tolerance vs bilateral tolerance

A bilateral tolerance allows a tolerance in both directions.

A unilateral tolerance allows a tolerance in only one direction.

A bilateral tolerance is plus AND minus. A unilateral tolerance is a plus OR minus tolerance.

If we take the unilateral tolerance from the picture above and convert it to a bilateral tolerance it could be either:

• 39.75 +/- 0.25
• 39.7 +0.3/-0.2
• 39.6 +0.4/-0.1

Notice that the important feature of the tolerance is that it has both a positive and negative tolerance. Neither side of the tolerance is zero.

### Unilateral tolerance vs limit tolerance

A unilateral tolerance specifies a nominal size and a plus or minus tolerance. These values are used to determine the tolerance range for a feature.

A limit tolerance skips the calculation step and simply gives you the tolerance range. Instead of a nominal size and a tolerance, the top and bottom of the tolerance range are directly listed.

Let’s compare some unilateral and limit tolerances to see how they differ:

 Unilateral Tolerance Limit Tolerance 10.0 +0/-0.5 9.5 - 10.0 5.525 +0.025/+0.050 5.550 - 5.575 7.55 +0/+0.15 7.55 - 7.70 2.324 -0.005/-0.010 2.314 - 2.319

## What is a unilateral tolerance used for?

A unilateral tolerance is most often used to specify a tolerance associated with a specific fit such as a clearance fit or interference fit.