Analyze a Wolman pebble count, manual riprap count, or calibrated photograph measurements. The calculator returns D16, D30, D50, D84, D90, a cumulative percent-finer curve, material classification, and transparent checks against FHWA Classes I-X or custom project gradation limits.
Particle-size units
Existing physical-size entries are converted when units change.
What this analysis reports
Characteristic sizes
D16, D30, D50, D84 and D90, plus D5/D10/D15/D60/D85/D95/D100 for specification work.
Transparent comparisons
Every entered class limit or percent-finer envelope point is shown as an individual pass/fail check.
For educational purposes only. Not a substitute for professional engineering judgment.
How gradation analysis works
1. Measure consistently
For a field Wolman count, measure the intermediate or b-axis of each selected particle. Each observation has equal count weight.
2. Order and interpolate
Values are sorted from smallest to largest. Characteristic D-percentiles use inclusive linear interpolation between adjacent ordered measurements.
3. Compare limits
Select an FHWA HEC-23 Table 4.1 D50/D100 class screen, or supply controlling agency D15/D50/D85/D100 limits or a percent-finer curve envelope for point-by-point checks.
Count gradation versus sieve gradation
This tool reports an equal-count particle distribution. A laboratory sieve analysis usually reports percent passing by mass. Those distributions answer related but different questions and should not be substituted for one another unless the governing method explicitly allows it.
| Method | Weighting | Typical use |
|---|---|---|
| Wolman / riprap count | One vote per particle | Surface substrate and placed-rock screening |
| Laboratory sieve | Mass retained or passing | Aggregate and soil material specifications |
Frequently asked questions
What do D16, D50, and D84 mean?
Dx is the particle size for which x percent of the measured sample is finer. D50 is the median size. D16 and D84 describe the spread around that median and are commonly used in streambed and sediment analyses.
How many particles should a Wolman pebble count include?
A traditional field count commonly uses about 100 spatially distributed particles. The calculator can analyze smaller samples, but it flags them because a small or selectively collected sample can give unstable percentiles.
Can the calculator automatically measure stones in a photograph?
No. Automatic segmentation can be unreliable when particles overlap, shadows vary, or scale and perspective are not controlled. The scaled-photo workflow instead asks for a known pixel scale and explicitly measured particle axes, making every conversion reviewable.
Which FHWA standard riprap classes are included?
FHWA Classes I through X are preloaded from HEC-23 Volume 2, Table 4.1. The built-in screen uses each class's nominal D50, allowable D50 range, and D100 maximum. Because Table 4.1 is not a complete D15/D85 or percent-finer acceptance envelope, agency and project-specific limits can still be entered separately.
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Last verified: August 2026