Compression Strength Paper Testing Methods

There are two ways of measuring the compression strength, the Ring Crush Test (RCT) and the Short-Span Compressive Test (SCT).

Calculate and Understand Compression Strength in Paper Products

Understanding how paper products perform under compression is essential for ensuring packaging strength and reliability. On this page, you’ll find key insights into the Ring Crush Test (RCT), the Short Span Compressive Test (SCT), and the compression strength formula. Use our interactive calculator to apply these formulas directly and estimate your paper’s compressive strength quickly and accurately.

Ring Crush Test

The conversion will also change with basis weight. It is speculated that this dependency of basis weight is caused by a tendency of samples to buckle in the RCT because of a longer span length than the SCT, viz. 12 mm and 0.7 mm respectively.

The RCT also depends on the quality of specimen preparation. Only an exact parallelism of the specimen and guides guarantees an exact determination of the Ring Crush Strength. Thus, the SCT frequently replaces it.

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Short Span Compressive Test

The material characteristic "compressive strength" can be determined with high accuracy using SCT because of the buckling stability provided by short test lengths over traditional testing methods (Ring Crush Test, Corrugated Crush Test or Linear Crush Test).

Due to these technical advantages, the SCT is being used more and more worldwide. Thus qualified paper testing laboratories recommend using the SCT to predict the final characteristics of boxes.

For those users still not very familiar with this property, it is possible to calculate back the RCT value knowing the value of the SCT. However, there are some important limits to this practice, as highlighted above, which should be taken into serious consideration.

The compression strength formula

The formula is the equation of a linear regression of the RCT and SCT values of samples with different basis weights. Lorentzen and Wettre provide a general equation which should be considered as a standard example and not used directly without a careful check of the parameters.

  • RCT CD= 120 x SCT CD - 97 or approximately
  • RCT CD= 95 x SCT CD (N)

The equation is specific for each paper machine. This means that the equation should be agreed upon by producer and users based on a series of dedicated tests.

Apply these formulas directly in our calculator below

Ring Crush Test (RCT) Calculator

How this works
This calculator estimates the Ring Crush Test (RCT CD) value from the Short Span Compressive Test (SCT CD) value.

- Equation 1: RCT CD = 120 × SCT CD – 97
- Equation 2: RCT CD = 95 × SCT CD

Select the equation that best fits your test method or local guidelines.

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FAQ

Compression strength refers to a paper’s ability to resist crushing forces. It’s a key measure of packaging performance, ensuring boxes can stack and transport safely without collapsing.

The Short Span Compression Test (SCT) measures the compressive strength of paper over a short span. It’s used because it provides consistent results and avoids buckling, giving a reliable indicator of paper performance. 

The Ring Crush Test (RCT) measures compression strength using a longer span of paper that can buckle more easily. It’s been largely replaced by the SCT because SCT results are more consistent and less prone to buckling variations. 

Compression strength can be estimated using formulas that relate SCT to RCT values. Common equations include RCT CD = 120 x SCT CD - 97 or RCT CD = 95 x SCT CD. Always check with your supplier for the best equation for your specific product. 

RCT stands for Ring Crush Test, a standard test for measuring the compressive strength of paperboard.

Yes, we now offer an interactive calculator that allows you to estimate RCT values from SCT results, using the recommended formulas. 

The compressive strength of a corrugated box refers to its ability to withstand crushing forces during stacking and transportation. It’s influenced by factors like material type, flute profile, and construction, and can be estimated using standard tests like RCT and SCT.