Calculator tools

Stormwater Detention PRE-POST Calculator

Compare pre-development and post-development stormwater flows, and size the detention storage needed to bring your site back within limits.

What this tool does

Work out your detention requirement

Development increases impervious area, which increases peak stormwater runoff. This calculator compares the pre-development and post-development flows for your site and works out the detention volume required to hold the difference back to an acceptable rate.

Support your consent application

Built for engineers, developers, and specifiers who need a defensible starting point for stormwater design. Use it early to test whether a site works, size the storage before selecting a tank, and reduce back and forth during the consent process.

Trade tool

Trade account required

This calculator is available to Promax trade customers who have been granted tool access. A trade account is free and takes about a minute. You just need your email, no password.

  1. Create your free trade account Tell us your name and company. Access is instant and also unlocks technical downloads across the site.
  2. Come back here and request access Once you are signed in, this page shows a one-click request. We will switch the calculator on for your account.
Create free trade account

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Talk to our team.

Our team knows this range inside out. Tell us about your build, what you're trying to achieve, where it's going, or what you're unsure about and we'll come back with a clear recommendation. No jargon, no obligation.

  • Personalised product recommendations for your specific project
  • Spec, compliance and sizing guidance included
  • Fast replies — usually within one business day
Phone 0800 77 66 29
Trusted by NZ homeowners, builders and specifiers since 1992.