Two free sites, then USD 100 per site.

Quick checks are free

A quick check works out runoff for areas you type in, without saving a site. It is free, needs an account, and has a daily limit only to stop abuse. Its report is marked "Quick check: not a site record, not for submission": what you pay for is the saved site, with its record of every input, change and reason that a plan reviewer relies on.

What every site includes

• Your reports stay yours. Reports and published versions stay available after the working window closes.
• USD 20 of AI assistant use. More is available in USD 5 top-ups. After the allowance is used, answers come from a lighter AI model, which may give shorter and less thorough answers, up to twice the allowance; then answers stop on that site until a top-up is bought. The assistant panel shows what is used in dollars and as a percentage.

CurveNumber works for sites in the contiguous United States (the lower 48 states). Alaska, Hawaii, Puerto Rico and the territories are not covered yet.

What it is for, and what it is not

CurveNumber computes runoff volume and peak discharge for a site's existing and proposed conditions by the NRCS methods (TR-55 and NEH-630), checks stormwater practices against them, and writes a report a plan reviewer can follow. It is a runoff and peak calculator with practice checks. It is not a full stormwater design package, and it does not replace one where a project needs:

• a site outside the contiguous United States;
• detention design from scratch: CurveNumber routes the practices you size; it does not size a pond for you or route a network of connected basins.

Peaks use the NRCS standard unit hydrograph, or the NRCS Delmarva unit hydrograph (peak rate factor 284) on the Delmarva coastal plain and where a local rule asks for it; every peak names the one it used. Design rainfall comes from NOAA Atlas 14, and in Washington and Oregon from NOAA Atlas 2 (1973), for 24 hour storms of 2 to 100 years; type other durations and return periods from the published source.

How you pay

During the beta, payment is by invoice; card payment is not offered yet. When a site, a re-open or an AI top-up is not covered by your balance, the app offers to send an invoice for it, and it is added once the invoice is paid.

Add funds. On the Billing screen you can also put money on your account balance ahead of time, by invoice. The balance pays for sites, re-opens and AI top-ups automatically, and a receipt is emailed for every use. In the words the Add funds screen shows before any payment: Money you add stays on your CurveNumber balance until you spend it. It never expires, it is not refundable, and it can only be used on CurveNumber, for sites, re-opens and AI top-ups. Sales tax, where it applies, is charged when the balance is spent, not when you add funds.