Window U-Value Calculator
The Window U-Value Calculator estimates how much heat your existing windows lose, models the savings from upgrading to A-rated double glazing or triple glazing, and gives a payback period at 2026 gas rates. U-value (W/m²K) measures heat loss — lower is better.
Window U-Value Calculator
Compare glazing options and calculate heat loss savings.
See 20 real-world examples
Each link below runs this calculator with real UK city and property inputs and publishes the exact computed savings, payback time, install cost and CO2 figures.
- Does Secondary Glazing Save Energy in Glasgow?
- Double Glazing U-Value Calculator for Edinburgh Homes
- How Much Can You Save by Upgrading Your Windows?
- How Much Heat Does Your Secondary Glazing Save?
- Secondary Glazing U-Value Calculator
- Secondary Glazing U-Value Calculator for Brighton
- Secondary Glazing U-Value Calculator for Leeds Homes
- Triple Glazing U-Value Calculator for Belfast
- Triple Glazing U-Value Calculator for Bristol
- Triple Glazing U-Value Calculator for Liverpool Homes
- Triple Glazing U-Value Calculator for Manchester
- Triple Glazing U-Value Calculator for Nottingham
- Triple Glazing U-Value Calculator for Southampton
- Triple Glazing U-Value Calculator for Swansea
- Window U-Value Calculator Double Glazing in Leicester
- Window U-Value Calculator Double Glazing in London
- Window U-Value Calculator for Newcastle Homes
- Window U-Value Calculator for Plymouth
- Window U-Value Calculator for Stoke-on-Trent Homes
- Window U-Value Calculator for Your Cardiff Home
How this calculator works
Annual heat loss through windows is U-value × area × heating-degree-days × 24 / 1000, multiplied by a BRE utilisation factor of 0.6 to account for solar gain and intermittent heating. We use 2,200 K·days as the UK average. Savings convert kWh × 7.2p gas rate / 0.9 boiler efficiency. Per-window costs are based on FENSA installer averages: £450 (A-rated double), £650 (triple), £850 (premium triple).
Sources & references
Frequently asked questions
How much do new windows save per year?
For an average 3-bed UK home upgrading from single to A-rated double glazing, expect to save £80-£200 a year on heating. The Energy Saving Trust's figures are similar. Triple glazing adds another £30-£60 in savings — usually not enough to justify the cost premium unless noise reduction matters.
Is the payback really 15-25 years?
Yes — windows are a comfort, security and aesthetic upgrade as much as an energy one. The energy payback rarely justifies replacement on its own. The right comparison is usually "I need new windows anyway, so which tier is best value?"
What does U-value mean?
U-value measures how much heat (in watts) passes through one square metre of the window for every degree of temperature difference between inside and outside. Lower is better. Single glazing is around 5.0; modern A-rated double glazing is 1.2; triple glazing is 0.6-0.8.
Do I need planning permission for new windows?
Like-for-like replacements in non-listed, non-conservation-area properties are usually permitted development. Listed buildings, conservation areas and AONBs require listed-building consent or planning permission for any visible change.
What is FENSA?
FENSA is the Fenestration Self-Assessment Scheme — a UK government-authorised competent-person scheme. FENSA installers can self-certify that replacement windows meet Building Regulations Part L (energy efficiency) without requiring a separate Building Control inspection.
Tool: Window U-Value Calculator. Last reviewed: . Figures based on Ofgem price cap, gov.uk grant guidance, and Energy Saving Trust advice. Verify scheme eligibility with your installer before commissioning work.