Shim Thickness Calculation: Formula, Example and Chart

When it comes to aligning heavy machinery, levelling a structural base plate or correcting the valve clearances, the choice of shim thickness is not negotiable. Any change of a fraction of a millimetre could cause vibration, wear prematurely or even disastrous equipment failure. This guide describes what shim thickness is, the importance of shim thickness, the standard shim thickness formula, a step by step shim calculation example and a practical industrial shims thickness chart that takes into account all the ultra-thin 0.001 shim stock to the heavy-duty thick shims that are used in civil and offshore structures.

What Is Shim Thickness?

A shim is a small item of material between two mating surfaces to fill in gaps, to fix misalignment, or to provide a fit. The shim thickness is the closest dimension perpendicular to the shim face and is the most important parameter of a shim to select or manufacture.

Shims are made in a variety of thicknesses. The shims required by the fine-tolerance applications can be as thin as 0.025 mm (commonly sold as 0.001 shim stock in imperial measurement), whereas structural and civil applications frequently use 3mm shim, 4mm shim or heavier. Other forms of non-metallic shims like 2mm plastic shims are also applied in construction, glazing and electrical isolation tasks where metals would be inappropriate.

At Aashish Metal and Alloys, we supply precision stainless steel shims, brass shims, and copper shims in virtually any thickness, from ultra-thin foil to heavy-gauge plate, ensuring engineers always have the right material for the job.

Why Shim Thickness Calculation Matters

Selecting the correct shim thickness is not a matter of guesswork. Incorrect shimming produces consequences that range from minor inefficiencies to dangerous structural failures:

• Machinery misalignment leads to increased bearing loads, overheating, and shortened service life.
• Over-shimming introduces preload stresses that can crack housings or distort precision components.
• Under-shimming leaves residual gaps that allow movement, vibration, and fatigue cracking.
• In pipe systems and flanged joints, incorrect shim thickness compromises sealing surfaces and causes leaks.
• In structural steelwork, base plate shimming controls elevation and load transfer; errors here have serious safety implications.

 Precision tools such as feeler gauges, micrometers, and dial indicators (sometimes complemented by inspection devices like elcometer shims used for coating thickness calibration) are used to measure existing gaps accurately before any shim calculation is performed.

Shim Thickness Formula

The core shim thickness formula for the most common industrial application, closing a measured gap to a desired clearance, is:

Where:

• T = thickness of the shim (or total stack thickness if using multiple shims)
• G = the current gap or clearance measured between the two mating surfaces
• C = the specified or desired clearance after shimming

For valve shim adjustment, the formula is extended to account for the existing shim already fitted:

For machine alignment using a dial indicator reading at a known offset distance, a proportional formula applies:

When using shim stacks with multiple layers rather than a single piece, the individual shim thicknesses simply add together: Total Stack Thickness = T1 + T2 + T3 + … This stacking approach is common when 2mm shim stock or other standard sizes are combined to achieve a non-standard total thickness.

Shim Calculation Example

The following step-by-step shim calculation example applies to aligning a centrifugal pump base plate in an industrial plant.

Scenario

A pump base plate is found to sit 2.65 mm lower on one side than the required level. The specification demands a maximum residual gap of 0.10 mm after shimming. A stainless steel 316/316L shim is to be used to correct this because of its corrosion resistance in a wet environment.

Step 1: Measure the Gap

Using a feeler gauge, the measured gap (G) between the base plate and the foundation = 2.65 mm

Step 2: Identify the Target Clearance

The project specification allows a residual clearance (C) of 0.10 mm after shimming.

Step 3: Apply the Formula

Step 4: Select the Shim

A single shim of 2.55 mm is not a standard catalogue thickness. The engineer combines one 2mm shim stock plate plus one 0.50 mm shim to achieve a stack of 2.50 mm, resulting in a final gap of 0.15 mm, which is within the acceptable tolerance range of 0.10 mm to 0.20 mm. Alternatively, a custom ground shim of exactly 2.55 mm can be ordered for critical installations.

For corrosive environments, our Stainless Steel 316/316L Shims offer the ideal combination of precise dimensional tolerance and long-term corrosion resistance. For less demanding environments, our Stainless Steel 304/304L Shims are a cost-effective alternative.

Industrial Shims Thickness Chart

The table below provides a comprehensive industrial shims thickness chart covering the full range from precision foil to heavy structural grades. Use this chart as a starting reference when selecting shims for your application.

Standard shim thicknesses from 0.05 mm through to 6 mm (and beyond for custom heavy fabrications) are stocked and available for rapid dispatch. For a full overview of material grades and form factors, visit our dedicated stainless steel shims supplier page.

Choosing the Right Shim Material for Your Thickness Requirement

Material selection works hand in hand with thickness selection. A very thin 0.001 shim stock in stainless steel offers rigidity and durability, while the same thickness in brass allows easier cutting and forming. 2mm plastic shims provide electrical isolation and resist chemicals that would attack metals. Thick shims above 4 mm are most commonly specified in carbon steel or stainless steel because they need to carry significant compressive loads.

For a detailed breakdown of which grade and finish to specify, refer to our expert guide on stainless steel shim materials, uses and types. Grades such as SS 304/304L and SS 316/316L are the most widely specified across oil and gas, chemical processing, marine, and food manufacturing industries.

Practical Tips for Accurate Shim Thickness Calculation

• Always measure the gap at multiple points and take the average value to account for surface irregularities.
• Use a calibrated micrometer or digital calliper to verify the actual thickness of each shim before installation, as rolled stock can vary slightly from its nominal value.
• When stacking shims, limit the number of layers. A stack of more than three shims can introduce cumulative tolerance errors and reduce long-term stability.
• For valve and engine applications, always cross-reference your calculated thickness with the manufacturer’s available shim size increments (typically 0.025 mm steps).
• Record all measurements and shim thicknesses used so that future maintenance checks can reference a known baseline.

The Bottom Line

Accurate shim thickness calculation is a fundamental engineering discipline that underpins machine reliability, structural integrity, and process safety. By applying the correct shim thickness formula and referencing a reliable industrial shims thickness chart, engineers can consistently achieve the precise fits that modern industrial equipment demands.

Aashish Metal and Alloys stocks an extensive range of precision shims across all materials and thicknesses, from ultra-thin foil to heavy thick shims for structural use. Contact our technical team today to discuss your specific requirements and request a quote for standard or custom dimensions.

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