Standard: ASTM D422 / BS 1377 Part 2 using 151H hydrometer in a 1000 mL cylinder.

Step 1 — Reading conversion: If the user enters 1.029, convert: R = (1.029 − 1) × 1000 = 29. If entered as 29, use directly.

Step 2 — Meniscus correction:

Step 3 — Temperature & zero correction:

Step 4 — Temperature correction factors (Ct):

T (°C)	Ct	T (°C)	Ct
15	−1.1	23	0.7
16	−0.9	24	1.0
17	−0.7	25	1.3
18	−0.5	26	1.65
19	−0.3	27	2.0
20	0.0	28	2.5
21	0.2	29	3.05
22	0.4	30	3.8

Step 5 — Effective depth L (cm): Based on calibration of 151H hydrometer in 1000 mL cylinder:

Step 6 — Viscosity factor K: Stokes' law constant combining water viscosity and particle density:

Simplified as K ≈ 0.013 for Gs=2.68 at 25°C. Full K table (ASTM D422 Table 1) is interpolated by the engine.

Step 7 — Particle diameter D (mm):

Step 8 — % Finer P (raw hydrometer):

Step 9 — Adjusted % Finer Pa (CRITICAL): The hydrometer test is run only on the fines fraction. To express results relative to the total original sample:

Where P₇₅ is the % passing the 75 µm (No. 200) sieve from the sieve analysis. This adjustment is mandatory for plotting a combined gradation curve.