How to use the SPT N-value correction calculator
A raw field blow count from ASTM D1586 depends on the hammer, the rod string, the borehole and the depth of the test. Before the number can be used in design it has to be normalized. This calculator computes N60 = N · CE · CB · CS · CR for the equipment corrections, then multiplies by the overburden factor CN = √(2000/σ'v in psf), capped at 1.7, to give (N1)60 referenced to about 1 tsf.
CE is the measured hammer energy ratio divided by the 60 percent reference. US drilling practice covers a wide spread: donut hammers deliver roughly 45 percent, safety hammers about 60 percent, and automatic trip hammers 80 percent or more, so the same soil can return very different raw counts from crew to crew. CR accounts for energy lost in short rod strings, and the water table matters because pore pressure lowers effective stress and raises CN.
| (N1)60 | Relative density |
|---|---|
| 0 to 4 | Very loose |
| 4 to 10 | Loose |
| 10 to 30 | Medium dense |
| 30 to 50 | Dense |
| Over 50 | Very dense |
Corrected blow counts feed relative density estimates, liquefaction triggering charts and empirical bearing and settlement correlations. Blow counts are unreliable in gravel, cobbles and soft clay, so read the corrected value alongside the rest of the boring log and have a licensed geotechnical engineer interpret it for design.
Frequently asked questions
N60 normalizes the field blow count to 60 percent hammer energy and corrects for rod length, borehole diameter and sampler type. (N1)60 adds the overburden correction so the value is referenced to an effective stress of about 1 tsf. Relative density and liquefaction charts use (N1)60.
At shallow depth the effective stress is tiny, so CN grows without bound and would inflate the corrected blow count. Common practice caps CN near 1.7, and this calculator applies that same limit.
Blow counts depend on the energy actually delivered to the rods. US practice sees roughly 45 percent for donut hammers, 60 percent for safety hammers and 80 percent or more for automatic trip hammers, so values must be converted to the 60 percent reference before they can be compared.