Subgrade Modulus Conversion Calculator
Free Subgrade modulus conversion Calculator for civil engineering projects. Enter dimensions to get material lists and cost estimates.
Reviewed by Abdullah, Technical Content Specialist
Formula
k (MN/m3) = k (pci) x 0.2714 | k (pci) = CBR x 26
The modulus of subgrade reaction k can be converted between unit systems using direct multiplication factors. The empirical CBR-to-k correlation uses k (pci) = 26 x CBR for CBR values up to 10, with a decreasing slope for higher CBR values. The 30-inch plate k-value is approximately half of the 12-inch plate k-value.
Worked Examples
Example 1: Convert pci to MN/m3
Problem:Convert a subgrade modulus of 150 pci to MN/m3.
Solution:k (MN/m3) = k (pci) x 0.2714\nk = 150 x 0.2714\nk = 40.7 MN/m3
Result:150 pci = 40.7 MN/m3
Example 2: Estimate k from CBR
Problem:Estimate the subgrade modulus for a soil with CBR = 8.
Solution:k (pci) = 26 x CBR\nk = 26 x 8\nk = 208 pci\nk (MN/m3) = 208 x 0.2714 = 56.4 MN/m3
Result:Estimated k = 208 pci (56.4 MN/m3)
Frequently Asked Questions
What is the modulus of subgrade reaction (k-value)?
The modulus of subgrade reaction, commonly called the k-value, measures the pressure required to produce a unit deflection of a soil subgrade surface. It is expressed in pounds per cubic inch (pci) or MN/m3 and is determined by plate load testing in the field. The k-value is a critical input for rigid pavement design, as it characterizes the support provided by the subgrade and subbase layers beneath a concrete slab. Higher k-values indicate stiffer, more supportive soils.
What are the most common unit conversion mistakes?
Common errors include confusing fluid ounces with weight ounces, mixing up miles and nautical miles, forgetting that UK and US gallons differ (UK is 20% larger), using the wrong temperature formula, and not accounting for the difference between troy and avoirdupois ounces.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy