Question 1

How do I calculate a composite curve number?

Accepted Answer

Area-weight the CN of each land-use / hydrologic-soil-group subarea: CNw = Σ(CNi · Ai) / ΣAi. Delineate the drainage area into subareas of uniform cover and soil group, look up each subarea's CN from TR-55 Table 2-2, multiply by its area, sum, and divide by the total area. Round the result to the nearest whole number for use in the runoff equation.

Question 2

When is area-weighting the CN not appropriate?

Accepted Answer

When impervious and pervious areas respond very differently — especially with unconnected impervious area or a low pervious CN — a single weighted CN under-predicts runoff from small storms, because the runoff equation is nonlinear in CN. TR-55 Chapter 2 recommends computing runoff separately for each subarea (or using Figure 2-3/2-4 adjustments for unconnected impervious area) when the pervious CN is below about 60 or when impervious area is unconnected.

Question 3

What CN do I use for impervious surfaces?

Accepted Answer

CN = 98 for paved parking, roofs, and driveways, regardless of soil group — the soil is sealed off. Gravel streets are 76–91 and dirt streets 72–89 depending on hydrologic soil group (TR-55 Table 2-2a).

Question 4

Do I round the composite CN?

Accepted Answer

Standard practice (and TR-55 worksheets) round the weighted CN to the nearest whole number before computing S and runoff. The difference between CN 84.3 and 84 is well inside the uncertainty of the CN method itself. This calculator reports both the unrounded and rounded values.

Question 5

What are S and Ia?

Accepted Answer

S is the potential maximum retention after runoff begins: S = 1000/CN − 10 (inches). Ia is the initial abstraction — interception, infiltration before ponding, and surface storage — conventionally Ia = 0.2·S in TR-55. Runoff Q = (P − 0.2S)² / (P + 0.8S) for P > 0.2S, else Q = 0.

Question 6

Which hydrologic soil group should I use?

Accepted Answer

Use the NRCS Web Soil Survey (or a county soil survey) to map the soil series on the site, each of which carries an assigned hydrologic soil group A (high infiltration, sandy) through D (very slow, clayey or shallow). Dual classifications like B/D mean B if drained, D if undrained. For disturbed/compacted urban fill, many reviewers require dropping one group (e.g., B to C).

Cover type	A	B	C	D
Paved parking, roofs, driveways	98	98	98	98
Open space, good condition (>75% grass)	39	61	74	80
Open space, fair condition (50–75% grass)	49	69	79	84
Residential, ¼-acre lots (38% imperv.)	61	75	83	87
Residential, ½-acre lots (25% imperv.)	54	70	80	85
Commercial/business (85% imperv.)	89	92	94	95
Industrial (72% imperv.)	81	88	91	93
Woods, good condition	30	55	70	77
Meadow, continuous grass	30	58	71	78

Composite Curve Number Calculator

Common curve numbers (TR-55 Table 2-2a excerpt)

Worked example

Example — 25-acre mixed development

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Composite Curve Number Calculator

Common curve numbers (TR-55 Table 2-2a excerpt)

Worked example

Example — 25-acre mixed development

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