Runs in your browser. No account, no subscription — £25 +VAT only when you export a PDF.
Pay for the reports, not the software
Most psi-value software is sold as a monthly or annual licence — you pay to keep access whether you calculate one junction that month or fifty. ΨMonkey flips that around. The calculator is free to use: build any junction on the grid, solve it to BS EN ISO 10211, and read the Ψ-value and fRsi on screen for nothing. You only pay £25 +VAT (£30 inc VAT), once, when you export the Building Control-ready PDF for a junction.
When pay-per-junction wins
If you run psi calculations in volume every week, a licence might make sense. But a lot of people don't — an architect checking a detail before it's built, a developer chasing a marginal Part L pass on one plot, an assessor who needs the occasional better-than-default junction value. For them, a recurring fee is mostly dead time. Paying £25 +VAT only for the junctions you actually export is usually far cheaper over a year — and there's nothing to remember to cancel.
The same recognised method
Cheaper pricing doesn't mean a weaker calculation. ΨMonkey runs a two-dimensional finite-difference solver to BS EN ISO 10211 — the method required for calculated junction Ψ-values in SAP 10 and the Home Energy Model — and reports the fRsi temperature factor for condensation risk with every model. The PDF documents the model, materials and boundary conditions for Building Control. See a sample report or read how thermal bridging works.
No account, no lock-in
There's no sign-up to use ΨMonkey and nothing stored on our servers to hold you in. Model what you need, pay per junction if and when you export, and walk away whenever you like. If you'd rather hand the whole job over, our SAP team calculates junction Ψ-values as part of full assessments.
Frequently asked questions
Is ΨMonkey really free, with no subscription?
Yes. Modelling junctions and reading off the Ψ-value and f_Rsi temperature factor is free in your browser, with no account and no recurring fee. You only pay £25 +VAT (£30 inc VAT), once, when you export a Building Control-ready PDF for a junction.
How does pay-per-junction compare to a monthly licence?
If you calculate psi values every week, a monthly licence may suit you. If you calculate them occasionally — the odd junction on a project, a one-off detail to chase a Part L pass — paying £25 +VAT only for the reports you export is usually far cheaper, and there is nothing to remember to cancel.
What do I get for the £25 +VAT?
A Building Control-ready PDF for that junction: the model, the materials, the boundary conditions, the calculated Ψ-value and the f_Rsi temperature factor, documented to BS EN ISO 10211. You can re-export the same junction free for 24 hours.
Is the calculation method the same as paid software?
ΨMonkey uses a two-dimensional finite-difference solver to BS EN ISO 10211 — the standard required for calculated junction Ψ-values in SAP 10 and the Home Energy Model. The method is the recognised one; the difference is the pricing.
Ready? Open the main ΨMonkey calculator, see how it compares as a free alternative to AutoPSI, or jump to a specific junction.