Intensity-Duration-Frequency (IDF) curves relate rainfall intensity to storm duration for a set of return periods at one location. The Rational Method needs one intensity from them: the intensity for a storm as long as the time of concentration, at the design return period.
What Is an IDF Curve?
An IDF curve is a graph (or set of equations) that shows:
- Intensity (I): Rainfall rate, typically in inches per hour (in/hr) or millimeters per hour (mm/hr)
- Duration (D): Length of the storm or averaging period (minutes or hours)
- Frequency (F): Return period — how often the event is expected to occur (e.g., 10-year, 100-year)
For any given location, shorter storms have higher intensities, and rarer storms (longer return periods) have higher intensities than more common storms of the same duration. The intensity is an average over the whole duration, not the peak rate within the storm.
How to Read an IDF Curve
- 2-yr
- 5-yr
- 10-yr
- 25-yr
- 50-yr
- 100-yr
Step 1: Choose the design return period (e.g., 10-year for a minor storm drain).
Step 2: Calculate the time of concentration (Tc) for your drainage area. The design storm duration equals Tc.
Step 3: Find the curve for your return period on the chart above.
Step 4: Enter the chart at your duration on the horizontal axis, go up to the curve, then across to the intensity on the vertical axis. The dashed line shows it for Tc = 15 min on the 10-year curve: i = 4.60 in/hr.
Step 5: Read the number from the table rather than the graph when you can. Between tabulated durations, interpolate on log-log scales (the straight segments on the chart).
Important: IDF data is location-specific. The 10-year, 15-minute intensity is 6.52 in/hr at Miami International Airport, 4.60 in/hr at the Atlanta airport and 2.00 in/hr at Boise Airport (NOAA Atlas 14 point estimates, partial-duration series), so Miami's is more than three times Boise's. Always use data for your project location.
IDF Table: Hartsfield-Jackson Atlanta International Airport
NOAA Atlas 14 intensities (in/hr) at 33.6301° N, 84.4418° W, partial-duration series, from the PFDS on 2026-09-24. This is one point, shown as an example: look up your own site.
| Duration | 2-yr | 5-yr | 10-yr | 25-yr | 50-yr | 100-yr |
|---|---|---|---|---|---|---|
| 5 min | 5.46 | 6.68 | 7.74 | 9.23 | 10.4 | 11.6 |
| 10 min | 4.00 | 4.90 | 5.66 | 6.76 | 7.62 | 8.51 |
| 15 min | 3.25 | 3.98 | 4.60 | 5.49 | 6.20 | 6.92 |
| 30 min | 2.38 | 2.91 | 3.37 | 4.01 | 4.52 | 5.04 |
| 1 hour | 1.54 | 1.88 | 2.18 | 2.61 | 2.96 | 3.32 |
| 2 hours | 0.947 | 1.16 | 1.34 | 1.61 | 1.83 | 2.06 |
| 3 hours | 0.701 | 0.852 | 0.988 | 1.19 | 1.36 | 1.54 |
| 6 hours | 0.425 | 0.511 | 0.590 | 0.709 | 0.810 | 0.918 |
| 12 hours | 0.258 | 0.307 | 0.352 | 0.420 | 0.477 | 0.538 |
| 24 hours | 0.156 | 0.187 | 0.215 | 0.255 | 0.288 | 0.323 |
IDF Equation Forms
Many state and local drainage manuals publish IDF curves as fitted equations instead of charts, with t in minutes and i in in/hr (or mm/hr). The common forms are:
- i = a / (t + b)c — the general (Sherman) form used by the calculator below
- i = a / (t + b) — Talbot form (the general form with c = 1)
- i = a / tc — Bernard (power) form (the general form with b = 0)
Each return period has its own a, b and c, and the coefficients only apply over the durations they were fitted to. Fitting the general form to the Atlanta 10-year values from 5 minutes to 2 hours (least squares on log intensity) gives a = 37.35, b = 5.25, c = 0.685. At Tc = 15 min:
i = 37.35 / (15 + 5.25)0.685 = 4.76 in/hr
That is 3.4% above the tabulated 4.60 in/hr, which is the fit error, not a better answer. Use published coefficients when your agency requires them; otherwise read the table.
IDF Equation Calculator
Get the intensity for any duration from equation coefficients, or paste a NOAA Atlas 14 table to interpolate it and fit a, b and c for the return period you pick. Talbot and Bernard coefficients work too: enter c = 1 or b = 0.
Rainfall intensity
4.76 in/hr (121 mm/hr)
i = 37.35 / (15 + 5.25)^0.685
Peak flow Q = (121/120) C i A
15.4 cfs
Cf = 1.0 (no frequency factor). Some agencies raise C for 25- to 100-year storms; HEC-22 notes FHWA does not endorse that factor.
Open this in the Rational Method calculatorThis calculator only evaluates the numbers you give it. Take design intensities for your site from the NOAA Atlas 14 Precipitation Frequency Data Server or the IDF source your reviewing agency requires. It never extrapolates beyond a pasted table.
Worked Example: Duration → Intensity → Peak Flow
A 4.0-acre paved commercial site near the Atlanta airport drains to one inlet. C = 0.80, Tc = 15 min, design storm 10-year.
- Duration. Set the storm duration equal to Tc: t = 15 min.
- Intensity. PFDS gives a 10-year, 15-minute depth of 1.15 in. Converting to intensity: i = 1.15 in ÷ (15/60) hr = 4.60 in/hr, the same value as the intensity table.
- Peak flow. Q = (121/120) × 0.80 × 4.60 × 4.0 = 14.8 cfs. The 121/120 factor converts acre-in/hr to ft³/s exactly, as the site's Rational Method calculator does; HEC-22 Eq. 4.1 rounds it to 1.0 and gives 14.7 cfs.
- Duration between table rows. If Tc were 20 min, interpolate on log-log scales between 4.60 in/hr at 15 min and 3.37 in/hr at 30 min: i = 4.60 × (3.37/4.60)ln(20/15)/ln(2) = 4.04 in/hr, so Q = 13.0 cfs.
For detention sizing across many storm durations, the Modified Rational calculator takes the same a, b, c coefficients or a duration-intensity table.
How to Get IDF Data from NOAA Atlas 14
- Open the NOAA Precipitation Frequency Data Server (PFDS) and pick your state.
- Enter the site latitude and longitude, search an address, or click the map.
- Set Data type to precipitation intensity (or keep depth and divide by the duration in hours), choose English or metric units, and choose the time series your agency uses. The default is the partial-duration series.
- Read the value at your duration and return period, or download “Estimates from the table in CSV format” (or copy the whole table, title row included) and paste it into the calculator above.
- Note the Atlas 14 volume and version printed above the table in your calculations.
The values in parentheses under each estimate are its 90% confidence interval. Some agencies ask for the upper bound for critical structures.
Where to Find IDF Data
NOAA Atlas 14 (Recommended where available)
The PFDS provides NOAA Atlas 14 point estimates for most US locations, for durations from 5 minutes to 60 days and return periods from 1 to 1,000 years, with 90% confidence intervals. Atlas 14 is published in 12 volumes:
| Volume | Coverage |
|---|---|
| 1 | Semiarid Southwest: Arizona, Nevada, New Mexico, Utah |
| 2 | Ohio River Basin and surrounding states: DE, DC, IL, IN, KY, MD, NJ, NC, OH, PA, SC, TN, VA, WV |
| 3 | Puerto Rico and the U.S. Virgin Islands |
| 4 | Hawaiian Islands |
| 5 | Selected Pacific Islands |
| 6 | California |
| 7 | Alaska |
| 8 | Midwestern states: CO, IA, KS, MI, MN, MO, NE, ND, OK, SD, WI |
| 9 | Southeastern states: AL, AR, FL, GA, LA, MS |
| 10 | Northeastern states: CT, ME, MA, NH, NY, RI, VT |
| 11 | Texas |
| 12 | Interior Northwest: Idaho, Montana, Wyoming (2024) |
| — | Washington and Oregon are not in Atlas 14. NOAA lists Arkell and Richards (1986) for durations under 1 hour, NOAA Atlas 2 (Vol. 9 Washington, Vol. 10 Oregon, 1973) for 1 to 24 hours and Technical Paper 49 beyond 24 hours. Most agencies there publish their own IDF curves. |
NOAA plans to publish NOAA Atlas 15, which will supersede Atlas 14, for the contiguous United States in 2027. Until your reviewing agency adopts it, design with Atlas 14.
State and Local Sources
Many state DOTs and local agencies publish IDF curves or equations specific to their jurisdictions. These may differ from NOAA Atlas 14 and may be required for regulatory compliance. Always check which data source your reviewing agency requires.
TP-40, HYDRO-35 and NOAA Atlas 2 — Superseded Where Atlas 14 Exists
Technical Paper 40 (1961), HYDRO-35 (1977, 5- to 60-minute rainfall in the eastern and central US) and NOAA Atlas 2 (1973, western states) were the predecessors of Atlas 14. Some older regulations still reference them; replace them with Atlas 14 values wherever Atlas 14 covers the site and your agency permits. NOAA Atlas 2 remains NOAA's listed source for Washington and Oregon.
Outside the United States
Use the national design-rainfall service: the Bureau of Meteorology Design Rainfall (IFD) data system in Australia, the Flood Estimation Handbook (FEH) rainfall model in the UK, and the Environment and Climate Change Canada IDF files in Canada.
How IDF Curves Are Built
For each duration, the largest rainfall totals in a gauge record (the annual maxima, or every event above a threshold for a partial-duration series) are fitted with a probability distribution. The depth for each return period is divided by the duration to give an average intensity, and plotting intensity against duration on log-log axes gives the near-straight curves in the chart. NOAA Atlas 14 applies regional frequency analysis (L-moments) to records from thousands of gauges, interpolates the results to a grid, and publishes a 90% confidence interval with every estimate.
Design Storm Selection
| Application | Typical Return Period | Annual Exceedance Probability |
|---|---|---|
| Roof / building drainage | 100-year | 1% |
| Minor storm drains | 10-year | 10% |
| Major storm drains | 25-year | 4% |
| Bridge / culvert waterway | 50 to 100-year | 2% to 1% |
| Floodplain management | 100-year | 1% |
Frequently Asked Questions
What does an IDF curve show?
An IDF curve shows the average rainfall intensity that a storm of a given duration reaches, on average, once per return period at one location. Each curve is one return period; intensity falls as duration gets longer and rises as the return period gets rarer.
Which duration do I read from the IDF curve for the Rational Method?
Use a storm duration equal to the time of concentration (Tc) of the drainage area. Many agencies also set a minimum Tc, often 5 to 10 minutes, so check your local criteria before reading the curve.
How do I convert a NOAA Atlas 14 depth to intensity?
Divide the depth by the duration in hours. At the Atlanta airport the 10-year, 15-minute depth is 1.15 in, so the intensity is 1.15 ÷ 0.25 hr = 4.60 in/hr. PFDS can also show intensity directly.
Should I use partial-duration or annual-maximum series values?
PFDS shows the partial-duration series (average recurrence interval) by default and offers the annual-maximum series (annual exceedance probability) as an option. The two differ mainly for frequent storms: at the Atlanta airport the 15-minute intensity is 3.25 in/hr (2-year PDS) against 3.03 in/hr (1/2 AEP), but 4.60 against 4.56 in/hr at 10 years. Use the series your reviewing agency specifies.
Does NOAA Atlas 14 cover every US state?
No. Atlas 14 covers every state except Washington and Oregon, plus Puerto Rico, the U.S. Virgin Islands and selected Pacific islands. For Washington and Oregon, NOAA lists Arkell and Richards (1986) for durations under 1 hour, NOAA Atlas 2 (1973) for 1 to 24 hours and Technical Paper 49 beyond 24 hours; state and local agencies there publish their own IDF curves.
Where do I get IDF data outside the United States?
Use the national design-rainfall service: the Bureau of Meteorology Design Rainfall (IFD) data system in Australia, the Flood Estimation Handbook (FEH) rainfall model in the UK, and the Environment and Climate Change Canada IDF files in Canada.
Will NOAA Atlas 15 replace Atlas 14?
Yes, once it is published. NOAA says Atlas 15 will supersede Atlas 14 as the national standard, with published estimates for the contiguous United States planned for 2027. Keep using Atlas 14 until your reviewing agency adopts Atlas 15.
Primary Sources
- NOAA (2004–2024). NOAA Atlas 14: Precipitation-Frequency Atlas of the United States, Volumes 1–12. National Weather Service, Hydrometeorological Design Studies Center. Example values: Volume 9, Version 2, via the Precipitation Frequency Data Server.
- NOAA Office of Water Prediction. Current precipitation frequency documents — the volume list and the documents that apply in each state and territory by duration.
- Federal Highway Administration (2024). Urban Drainage Design Manual, Fourth Edition. Hydraulic Engineering Circular No. 22, FHWA-HIF-24-006. Section 4.1.1 (IDF curves) and Section 4.2.2 (Rational Method, Eq. 4.1).
- Miller, J.F., Frederick, R.H. and Tracey, R.J. (1973). NOAA Atlas 2: Precipitation-Frequency Atlas of the Western United States. National Weather Service.
- Hershfield, D.M. (1961). Rainfall Frequency Atlas of the United States (Technical Paper No. 40). U.S. Weather Bureau.
- NOAA. NOAA Atlas 15 information page — status and schedule of the Atlas 14 replacement.