SPCC plans require secondary containment to accommodate the design storm rainfall event in addition to the largest tank volume. This calculator provides 25-year, 24-hour precipitation depths from NOAA Atlas 14 for locations across the United States and calculates the volume of water that would accumulate in your containment area during the design storm. Use this to verify that your containment has adequate capacity after accounting for precipitation displacement.
Select your facility location to look up the 25-year, 24-hour precipitation depth from NOAA Atlas 14. The calculator attempts to fetch live data directly from NOAA; if unavailable, it uses verified reference values.
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This calculator fetches live precipitation frequency data directly from NOAA’s Atlas 14 Precipitation Frequency Data Server (PFDS). When you select a location or enter coordinates, the calculator queries NOAA’s servers in real time to retrieve the most current published data for your facility’s exact location.
When you select a city from the dropdown or enter lat/lon coordinates, the calculator sends a request to NOAA’s PFDS API at hdsc.nws.noaa.gov. NOAA returns precipitation depths for multiple return periods (2-year through 1000-year) and durations (5-minute through 60-day). The calculator extracts the 25-year, 24-hour value — the standard used for SPCC containment sizing — and displays all available durations for reference.
NOAA periodically updates Atlas 14 as new rainfall data becomes available and statistical models are refined. By fetching live data rather than using hardcoded tables, this calculator always reflects NOAA’s most current published values. This ensures your SPCC containment calculations use the same data that regulators and certifying PEs reference.
For official SPCC plan documentation, you may want to generate a formal NOAA report for your facility. Visit the NOAA PFDS directly:
On the NOAA site, you can click your facility location on the map or enter coordinates to generate a full precipitation frequency table with 90% confidence intervals, which can be saved or printed for your SPCC plan files.
NOAA Atlas 14 covers the contiguous United States, Alaska, Hawaii, Puerto Rico, and US territories. Some remote areas may have limited station density, resulting in wider confidence intervals. The data server is maintained by NOAA’s Office of Water Prediction and is occasionally unavailable for maintenance — if this occurs, you can enter your rainfall depth manually using a previously obtained value.
NOAA Atlas 14 provides precipitation depths for return periods of 1, 2, 5, 10, 25, 50, 100, 200, 500, and 1,000 years. While 25-year is the SPCC standard, some states or local jurisdictions require different return periods. You can use the manual entry field on the Calculator tab to enter any value from NOAA’s full frequency table.
The 25-year, 24-hour storm is a statistical rainfall event that has a 4% probability of being equaled or exceeded in any given year. The “25-year” refers to the average recurrence interval, not a prediction of when it will occur. It represents a significant but not extreme rainfall event and is the industry standard for SPCC secondary containment precipitation calculations.
The definitive source is NOAA Atlas 14 (Precipitation Frequency Data Server) at hdsc.nws.noaa.gov/pfds. Enter your facility’s coordinates or click on the map to get point-specific precipitation frequency estimates for various return periods and durations. The values in this calculator are representative approximations — use NOAA Atlas 14 for your actual SPCC plan.
Multiply the total containment surface area (in square feet) by the design storm rainfall depth converted to feet. For example: a 30 ft × 20 ft dike (600 ft²) in Denver, CO with a 25-year, 24-hour depth of 3.6 inches (0.30 ft) would accumulate 600 × 0.30 = 180 ft³ = 1,347 gallons of rainwater. This volume must be subtracted from the containment’s gross capacity when checking SPCC compliance.
EPA’s SPCC rule (40 CFR 112) requires containment to accommodate the “most likely quantity of precipitation” but does not specify an exact return period. However, EPA guidance and industry practice have established the 25-year, 24-hour storm as the accepted standard. Some states or local jurisdictions may require different return periods (e.g., 100-year storm), so always check your local regulations.
The conservative approach is to use the full containment area without subtracting tank footprints. While tanks displace some rain, their tops also collect rain that runs off into the containment. For open-top or roofed tanks that divert rainfall outside the containment area, you may subtract that area — but document the justification in your SPCC plan.
Several states impose stricter requirements. Use the “enter manually” option in this calculator to input any rainfall depth. You can look up different return periods (10-year, 50-year, 100-year) on NOAA Atlas 14 by selecting the appropriate frequency column for your location.
Several approaches can reduce or eliminate precipitation displacement: install a roof or canopy over the containment area; use an automated pump-out system with oil-water separation; install a normally-closed drain valve with inspection protocol; or reduce the containment footprint by using taller walls. Each approach has regulatory implications that must be addressed in your SPCC plan and approved by the certifying PE where applicable.
Freeboard is the vertical distance between the top of the containment wall and the maximum expected liquid level. It provides a safety margin against overtopping from wave action, splashing, or unexpected volume. Common requirements range from 6 inches to 1 foot. Freeboard volume (area × freeboard height) should be subtracted from gross capacity in addition to precipitation and tank displacement.
Secondary containment and precipitation management involve ongoing tracking, inspection, and documentation. EHS software streamlines these requirements:
SPCC Plan Management - Maintain your complete SPCC plan digitally, including containment calculations, precipitation data, tank inventories, and facility diagrams. Track plan amendments and PE certification dates with automated reminders for the required 5-year review cycle.
Containment Inspection Scheduling - Schedule and document regular containment inspections including visual integrity checks, accumulated water removal, drain valve status, and liner condition. Track corrective actions for any deficiencies found with automatic notifications.
Precipitation Management Documentation - Log rainwater accumulation and removal events, including pre-discharge inspections confirming no visible sheen or discoloration before drain valve operation. Maintain the audit trail required by SPCC regulations for each discharge event.
Tank and Equipment Inventory - Centralize your tank database with dimensions, capacities, contents, containment assignments, and inspection histories. Generate reports for regulatory submissions and audit preparation.
Compliance Calendar - Track all SPCC-related deadlines: monthly containment inspections, annual integrity testing, 5-year plan reviews, Tier II reporting, and state-specific tank registration renewals. Never miss a deadline with automated reminders and escalation.
Spill Incident Tracking - Document spill events from initial notification through investigation, corrective actions, and regulatory reporting. Link incidents to specific tanks and containment areas for trend analysis.
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If you need help with containment rainfall calculations or SPCC compliance, please call us at (720) 547-5102.
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