What is the SCS runoff equation?

Q = (P − 0.2S)² / (P + 0.8S) with S = 1000/CN − 10, valid when P is greater than 0.2S. Below that, Q is zero. Source: TR-55 (1986) eq. 2-3 and 2-4, p. 2-1.

What CN does a paved parking lot take?

98 on every hydrologic soil group. Source: TR-55 Table 2-2a, p. 2-5.

What CN does lawn in good condition take on C soil?

74, for open space with grass cover over 75%. Source: TR-55 Table 2-2a, p. 2-5.

Can I use Ia = 0.05S?

Not with the TR-55 curve number tables. TR-55 and NEH 630 both say a ratio other than 0.2 needs a redeveloped set of curve numbers. Source: TR-55 p. 2-11; NEH 630 Ch. 10, p. 10-4.

What CN does NYSDEC require for pre-developed farmland?

Meadow. The design manual takes agricultural land as meadow in the pre-development condition. Source: NYS Stormwater Management Design Manual (July 31, 2024) § 4.6.

How much runoff does 6 in of rain give at CN 75?

3.28 in, TR-55's own worked example. Source: TR-55 Example 2-2, Fig. 2-6, p. 2-14.

Curve number and runoff calculator

Composite CN from cover type and hydrologic soil group, then TR-55 runoff depth for a 24-hr rainfall.

PRELIMINARYReference tables are still being checked against their printed sources. How results are checked

Storm

Rainfall P

24-hr depth for the storm.

Weighting

Subarea 1

Cover type

Hydrologic soil group

Area

Subarea 2

Cover type

Hydrologic soil group

Area

2.06in

2.06 in of runoff from 3.74 in of rain on composite CN 83.0.

Composite CN: 83.0
Potential retention S: 2.05 in
Initial abstraction Ia: 0.41 in
Total area: 4.00 ac
CN, subarea 1: 98
CN, subarea 2: 74

The link carries these inputs, not the answer.

Curve numbers by cover and soil group

The TR-55 rows the calculator looks CN up from, for average runoff condition and Ia = 0.2S. A dash means the table prints no value for that soil group.

Table 2-2a: Fully developed urban areas (18 rows)

Table 2-2a: Fully developed urban areas. TR-55 (1986) Table 2-2a, p. 2-5.
Cover	A	B	C	D
Open space (lawns, parks, golf courses, cemeteries, etc.), Poor condition (grass cover < 50%)	68	79	86	89
Open space (lawns, parks, golf courses, cemeteries, etc.), Fair condition (grass cover 50% to 75%)	49	69	79	84
Open space (lawns, parks, golf courses, cemeteries, etc.), Good condition (grass cover > 75%)	39	61	74	80
Impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of-way)	98	98	98	98
Streets and roads: Paved; curbs and storm sewers (excluding right-of-way)	98	98	98	98
Streets and roads: Paved; open ditches (including right-of-way)	83	89	92	93
Streets and roads: Gravel (including right-of-way)	76	85	89	91
Streets and roads: Dirt (including right-of-way)	72	82	87	89
Western desert urban areas: Natural desert landscaping (pervious areas only)	63	77	85	88
Western desert urban areas: Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2-inch sand or gravel mulch and basin borders)	96	96	96	96
Urban districts: Commercial and business	89	92	94	95
Urban districts: Industrial	81	88	91	93
Residential districts by average lot size: 1/8 acre or less (town houses)	77	85	90	92
Residential districts by average lot size: 1/4 acre	61	75	83	87
Residential districts by average lot size: 1/3 acre	57	72	81	86
Residential districts by average lot size: 1/2 acre	54	70	80	85
Residential districts by average lot size: 1 acre	51	68	79	84
Residential districts by average lot size: 2 acres	46	65	77	82

Table 2-2a: Developing urban areas (1 rows)

Table 2-2a: Developing urban areas. TR-55 (1986) Table 2-2a, p. 2-5.
Cover	A	B	C	D
Newly graded areas (pervious areas only, no vegetation)	77	86	91	94

Table 2-2b: Runoff curve numbers for cultivated agricultural lands (33 rows)

Table 2-2b: Runoff curve numbers for cultivated agricultural lands. TR-55 (1986) Table 2-2b, p. 2-6.
Cover	A	B	C	D
Fallow, Bare soil	77	86	91	94
Fallow, Crop residue cover (CR), Poor	76	85	90	93
Fallow, Crop residue cover (CR), Good	74	83	88	90
Row crops, Straight row (SR), Poor	72	81	88	91
Row crops, Straight row (SR), Good	67	78	85	89
Row crops, SR + CR, Poor	71	80	87	90
Row crops, SR + CR, Good	64	75	82	85
Row crops, Contoured (C), Poor	70	79	84	88
Row crops, Contoured (C), Good	65	75	82	86
Row crops, C + CR, Poor	69	78	83	87
Row crops, C + CR, Good	64	74	81	85
Row crops, Contoured & terraced (C&T), Poor	66	74	80	82
Row crops, Contoured & terraced (C&T), Good	62	71	78	81
Row crops, C&T + CR, Poor	65	73	79	81
Row crops, C&T + CR, Good	61	70	77	80
Small grain, SR, Poor	65	76	84	88
Small grain, SR, Good	63	75	83	87
Small grain, SR + CR, Poor	64	75	83	86
Small grain, SR + CR, Good	60	72	80	84
Small grain, C, Poor	63	74	82	85
Small grain, C, Good	61	73	81	84
Small grain, C + CR, Poor	62	73	81	84
Small grain, C + CR, Good	60	72	80	83
Small grain, C&T, Poor	61	72	79	82
Small grain, C&T, Good	59	70	78	81
Small grain, C&T + CR, Poor	60	71	78	81
Small grain, C&T + CR, Good	58	69	77	80
Close-seeded or broadcast legumes or rotation meadow, SR, Poor	66	77	85	89
Close-seeded or broadcast legumes or rotation meadow, SR, Good	58	72	81	85
Close-seeded or broadcast legumes or rotation meadow, C, Poor	64	75	83	85
Close-seeded or broadcast legumes or rotation meadow, C, Good	55	69	78	83
Close-seeded or broadcast legumes or rotation meadow, C&T, Poor	63	73	80	83
Close-seeded or broadcast legumes or rotation meadow, C&T, Good	51	67	76	80

Table 2-2c: Runoff curve numbers for other agricultural lands (14 rows)

Table 2-2c: Runoff curve numbers for other agricultural lands. TR-55 (1986) Table 2-2c, p. 2-7.
Cover	A	B	C	D
Pasture, grassland, or range - continuous forage for grazing, Poor	68	79	86	89
Pasture, grassland, or range - continuous forage for grazing, Fair	49	69	79	84
Pasture, grassland, or range - continuous forage for grazing, Good	39	61	74	80
Meadow - continuous grass, protected from grazing and generally mowed for hay	30	58	71	78
Brush - brush-weed-grass mixture with brush the major element, Poor	48	67	77	83
Brush - brush-weed-grass mixture with brush the major element, Fair	35	56	70	77
Brush - brush-weed-grass mixture with brush the major element, Good	30	48	65	73
Woods - grass combination (orchard or tree farm), Poor	57	73	82	86
Woods - grass combination (orchard or tree farm), Fair	43	65	76	82
Woods - grass combination (orchard or tree farm), Good	32	58	72	79
Woods, Poor	45	66	77	83
Woods, Fair	36	60	73	79
Woods, Good	30	55	70	77
Farmsteads - buildings, lanes, driveways, and surrounding lots	59	74	82	86

Table 2-2d: Runoff curve numbers for arid and semiarid rangelands (15 rows)

Table 2-2d: Runoff curve numbers for arid and semiarid rangelands. TR-55 (1986) Table 2-2d, p. 2-8.
Cover	A	B	C	D
Herbaceous - mixture of grass, weeds, and low-growing brush, with brush the minor element, Poor	—	80	87	93
Herbaceous - mixture of grass, weeds, and low-growing brush, with brush the minor element, Fair	—	71	81	89
Herbaceous - mixture of grass, weeds, and low-growing brush, with brush the minor element, Good	—	62	74	85
Oak-aspen - mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush, Poor	—	66	74	79
Oak-aspen - mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush, Fair	—	48	57	63
Oak-aspen - mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush, Good	—	30	41	48
Pinyon-juniper - pinyon, juniper, or both; grass understory, Poor	—	75	85	89
Pinyon-juniper - pinyon, juniper, or both; grass understory, Fair	—	58	73	80
Pinyon-juniper - pinyon, juniper, or both; grass understory, Good	—	41	61	71
Sagebrush with grass understory, Poor	—	67	80	85
Sagebrush with grass understory, Fair	—	51	63	70
Sagebrush with grass understory, Good	—	35	47	55
Desert shrub - major plants include saltbush, greasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus, Poor	63	77	85	88
Desert shrub - major plants include saltbush, greasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus, Fair	55	72	81	86
Desert shrub - major plants include saltbush, greasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus, Good	49	68	79	84

Worked example

The default inputs above are a published example: 1.5 ac paved parking and 2.5 ac lawn in good condition on HSG C (TR-55 Table 2-2a), at the 10-yr 24-hr depth of the DM2024 Ch. 7 example site (Table 7.1, 3.74 in).

CN = (98 × 1.50 ac + 74 × 2.50 ac) / 4.00 ac = 83.0
S = 1000 / 83.0 − 10 = 2.05 in
Ia = 0.2 × S = 0.41 in
Q = (3.74 − 0.41)² / (3.74 − 0.41 + 2.05) = 2.06 in

The formula

Q = (P − 0.2S)² / (P + 0.8S), S = 1000/CN − 10, for P > 0.2S

TR-55 (1986) eq. 2-3 and 2-4, p. 2-1

Q: direct runoff depth, in
P: rainfall depth for the 24-hr storm, in
S: potential maximum retention after runoff begins, in
Ia: initial abstraction, all losses before runoff begins (0.2S), in
CN: runoff curve number, from cover type and hydrologic soil group

Area-weighted CN or runoff-weighted Q

TR-55 Worksheet 2, WinTR-55 and WinTR-20 weight the curve number by area, and that is what a reviewer reproduces. NEH 630 Chapter 10 says weighting runoff is more exact where the curve numbers in one drainage area differ a lot, because a weighted CN under- or over-estimates Q depending on the storm. Pick the method your reviewer expects; both run through the same equation.

When the CN method stops working

TR-55 calls the procedure less accurate when runoff is under 0.5 in, says to use another procedure when the weighted CN is under 40, and does not cover snowmelt or rain on frozen ground (TR-55 p. 2-11). The calculator says so beside the answer when a run crosses one of those lines.

Questions about this calculation

What is the SCS runoff equation?: Q = (P − 0.2S)² / (P + 0.8S) with S = 1000/CN − 10, valid when P is greater than 0.2S. Below that, Q is zero.; TR-55 (1986) eq. 2-3 and 2-4, p. 2-1
What CN does a paved parking lot take?: 98 on every hydrologic soil group.; TR-55 Table 2-2a, p. 2-5
What CN does lawn in good condition take on C soil?: 74, for open space with grass cover over 75%.; TR-55 Table 2-2a, p. 2-5
Can I use Ia = 0.05S?: Not with the TR-55 curve number tables. TR-55 and NEH 630 both say a ratio other than 0.2 needs a redeveloped set of curve numbers.; TR-55 p. 2-11; NEH 630 Ch. 10, p. 10-4
What CN does NYSDEC require for pre-developed farmland?: Meadow. The design manual takes agricultural land as meadow in the pre-development condition.; NYS Stormwater Management Design Manual (July 31, 2024) § 4.6
How much runoff does 6 in of rain give at CN 75?: 3.28 in, TR-55's own worked example.; TR-55 Example 2-2, Fig. 2-6, p. 2-14

The other calculators

Time of concentration. TR-55 Tc from a sheet flow, a shallow concentrated flow and a channel flow segment, in hours and minutes.
Rational method peak flow. Peak runoff Q = Cf·C·i·A from the runoff coefficient, rainfall intensity and drainage area, with the frequency factor for the storm.
Detention volume estimate. Storage needed to hold post-development runoff to an allowable release rate, by the modified rational critical-duration method.
NY water quality volume. WQv = P·Rv·A/12 from the 90th percentile rainfall, impervious cover and contributing area, per the NYSDEC design manual.

How results are checked Pricing for the full app

The full app runs the whole site: Atlas 14 rainfall frozen for the project, pre and post hydrographs, and the pond routed through its outlets. Then the NYSDEC checks you select, and a PDF report with the SWMM file a reviewer can rerun.

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