TR-55 Peak Discharge Calculator

The NRCS TR-55 Graphical Peak Discharge method combines unit peak discharge, drainage area, runoff depth, and a pond and swamp adjustment: q_p = q_u × A_m × Q × F_p. Enter one homogeneous watershed’s characteristics to estimate its peak discharge.

Select one of the four NRCS 24-hour rainfall distribution types: I, IA, II, or III. Time of concentration T_c and the ratio of initial abstraction to rainfall, I_a/P, select unit peak discharge q_u. Calculations use US units internally; the unit switch converts the displayed inputs and results.

01 / INPUTS

Watershed & storm

All fields are required.

02 / RESULTS

Peak discharge

Results shown to 4 significant figures.

Estimated peak flow, q_pAwaiting calculation

—cfs

— m³/s

q_p = q_u × A_m × Q × F_p

Potential retention, S

— in

— mm

Initial abstraction, I_a

— in

— mm

I_a/P, computed

—

Used: —

Runoff depth, Q

— in

— mm

Unit peak discharge, q_u

—

csm/in = cfs/(mi² · in)

Pond & swamp factor, F_p

—

Nearest table percentage: —

Inspect table lookup & calculation

Enter valid inputs to inspect the Table F-1 lookup.

I_a/P row	C₀	C₁	C₂	q_u (csm/in)
No calculation yet.

—

T_c used: —

Internal US-unit inputs: —

—

03 / WORKFLOW

How to use the calculator

Choose your units.Use US units with area in square miles or acres, or SI units with area in square kilometres. Switching units converts the entered rainfall and area.
Describe the watershed.Enter drainage area, one curve number above 40 and at most 100, and a positive time of concentration in hours.
Define the storm and pond area.Enter the 24-hour rainfall, choose type I, IA, II, or III, and enter the percentage of pond and swamp area spread through the watershed.
Review the estimate and its limits.Read peak discharge in both unit systems. Check the warnings and open the calculation details to see the table rows and interpolation used.

04 / METHOD

Formulas used

S = 1000/CN − 10

I_a = 0.2S · r = I_a/P

Q = (P − I_a)² / (P + 0.8S), if P > I_a

Otherwise, Q = 0.

log₁₀(q_u) = C₀ + C₁ log₁₀(T_c) + C₂ [log₁₀(T_c)]²

q_p = q_u × A_m × Q × F_p

P, S, I_a, and Q are in inches; A_m is in mi²; T_c is in hours. Table F-1 gives the coefficients. Between rows, calculate q_u at each row, then interpolate q_u linearly in r.

Table 4-2 uses the nearest pond percentage: 0 → 1.00; 0.2 → 0.97; 1 → 0.87; 3 → 0.75; 5 → 0.72. A tie selects the lower percentage.

05 / QUESTIONS

Frequently asked questions

What does this calculator estimate?

It estimates peak discharge using the NRCS TR-55 Graphical Peak Discharge method (June 1986). It calculates peak discharge only, not hydrographs, routing, or TR-20.

How is unit peak discharge selected?

Rainfall type, time of concentration, and Ia/P select unit peak discharge from Table F-1. Between rows, the calculator interpolates unit peak discharge linearly, not the coefficients. Ia/P is limited to 0.10–0.50 and time of concentration to 0.1–10 hours, with warnings when a limit is used.

How are pond and swamp areas handled?

The adjustment factor Fp uses the nearest Table 4-2 percentage: 0, 0.2, 1, 3, or 5 percent. A tie uses the lower percentage. Above 5 percent, Fp is 0.72 with a warning. These pond and swamp areas must be spread through the watershed and not in the time-of-concentration flow path.

When does the calculator report no runoff?

When rainfall P is less than or equal to initial abstraction Ia, runoff depth Q and peak discharge qp are zero. Initial abstraction is calculated as Ia = 0.2S, where S = 1000/CN − 10 in inches.