Stratigraphic Thickness Calculator
What You Measure in the Field Isn't What's Actually There
Walk up to a tilted rock outcrop and measure straight down through a bed, and you'll get the apparent thickness — but unless that bed happens to be perfectly horizontal, that number overstates the bed's true thickness. The steeper the dip, the bigger the gap between what a tape measure reads and what the layer actually measures perpendicular to its own bedding planes. Correcting for that gap is a routine but essential step in stratigraphic fieldwork.
The Formula
At a dip of 0° (horizontal beds), cosine equals 1 and apparent thickness already equals true thickness. As dip increases toward 90° (vertical beds), cosine approaches 0, and a vertical measurement through the bed captures almost none of its true thickness — which is why dip angle has to be factored in every time.
Why This Correction Is Routine, Not Optional
- Stratigraphic column construction — building an accurate sequence of bed thicknesses across a field area requires every measurement to be corrected to true thickness first, or the whole column is skewed.
- Resource volume estimation — coal seams, ore bodies, and hydrocarbon-bearing units are valued by true thickness, since that's what actually determines the volume of material present.
- Cross-section accuracy — geologic cross-sections drawn from field measurements need true thickness to correctly represent subsurface geometry, not the raw, dip-inflated field reading.
- Comparing outcrops at different dips — two exposures of the same bed at different structural dips will show very different apparent thicknesses even though the bed itself hasn't changed; the correction is what makes them comparable.
How Dip Angle Shrinks Apparent Thickness
| Dip Angle | cos(Dip) | True Thickness (from 10m apparent) |
|---|---|---|
| 0° | 1.0000 | 10.00 m |
| 20° | 0.9397 | 9.40 m |
| 30° | 0.8660 | 8.66 m |
| 45° | 0.7071 | 7.07 m |
| 60° | 0.5000 | 5.00 m |
| 75° | 0.2588 | 2.59 m |
| 90° | 0.0000 | 0.00 m |
Computed directly from the formula above for a fixed 10 m apparent thickness; scale proportionally for any other apparent thickness value.
How to Use This Calculator
- Enter the Apparent Thickness measured in the field (e.g. meters).
- Enter the Bed Dip Angle in degrees, between 0 and 90.
- Select Calculate to get the true thickness, with the cosine correction shown explicitly.
Related Calculations
Estimate the volume of a corrected bed or sample with the Rock Volume Calculator, or work out fault movement across a sequence with the Fault Slip Rate Calculator.