United Kingdom version
How it works
Combine a quoted per-window replacement price with a conductive heat-loss comparison using whole-window U-values.
This is the United Kingdom planning view, with monetary inputs and outputs in GBP. A currency label does not establish that a tax rate, construction price, coverage rule or legal limit has been verified locally. The inputs and limitations below describe the actual scope.
Input query strings and outputs
Use an input name in this page’s URL as ?name=value, and join additional inputs with &. Portable shared links may instead use the compact ?ac= state parameter.
Input query strings
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?unitSystem=Measurement systemChoice · Optional · Default: metric · Accepted values: imperial, metric
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?windows=Number of windowsNumber · Required · Default: 10
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?windowArea=Average area of one windowNumber · Required · Default: 1.5
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?oldU=Existing window U-valueNumber · Required · Default: 2.8
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?newU=Proposed window U-valueNumber · Required · Default: 1.3
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?degreeDays=Local heating degree daysNumber · Required · Default: 2,500
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?heatEfficiency=Seasonal heating efficiencyPercentage · Required · Default: 90
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?energyRate=Heating energy cost per input kWhMoney · Required · Default: 0.1
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?unitRate=Material price per windowMoney · Required · Default: 0
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?laborCost=Total labor allowanceMoney · Required · Default: 0
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?deliveryCost=Delivery allowanceMoney · Required · Default: 0
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?otherCost=Other fixed costsMoney · Required · Default: 0
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?costMultiplier=Material/labor multiplierNumber · Required · Default: 1
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?spreadPercent=Low/high sensitivity around the entered totalPercentage · Required · Default: 15
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?taxRate=Explicit tax allowancePercentage · Required · Default: 0
Outputs
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totalAreaTotal window areaNumber
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annualInputSavingModeled heating input saved, kWhNumber
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annualSavingModeled annual heating savingCurrency · Currency: GBP
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materialCostMaterial costCurrency · Currency: GBP
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laborCostLabor allowanceCurrency · Currency: GBP
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deliveryCostDelivery allowanceCurrency · Currency: GBP
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otherCostOther specified costsCurrency · Currency: GBP
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taxEntered tax allowanceCurrency · Currency: GBP
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lowLow cost scenarioCurrency · Currency: GBP
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midMiddle cost scenarioCurrency · Primary output · Currency: GBP
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highHigh cost scenarioCurrency · Currency: GBP
Formula
heating input saved = window area × (old U − new U) × degree days × 24 ÷ (1,000 × heating efficiency).
Calculations retain numeric precision internally. Displayed values are rounded for readability. Quantities that must be purchased as whole units are rounded up only where the formula explicitly requires it.
Read the inputs before calculating
- Number of windows: Enter the quantity described by the label.
- Average area of one window: Enter the quantity described by the label. The measurement selector converts this physical quantity; it does not change the underlying scenario.
- Existing window U-value: Enter the quantity described by the label. The measurement selector converts this physical quantity; it does not change the underlying scenario.
- Proposed window U-value: Enter the quantity described by the label. The measurement selector converts this physical quantity; it does not change the underlying scenario.
- Local heating degree days: Enter the quantity described by the label. The measurement selector converts this physical quantity; it does not change the underlying scenario.
- Seasonal heating efficiency: Enter the quantity described by the label.
- Heating energy cost per input kWh: Enter the quantity described by the label.
- Material price per window: Zero means unpriced, not free. Enter a current, scoped supplier quote.
- Total labor allowance: Enter the quantity described by the label.
- Delivery allowance: Enter the quantity described by the label.
- Other fixed costs: Enter the quantity described by the label.
- Material/labor multiplier: Use a verified regional index only when supplied. 1 is an unadjusted scenario, not a researched index.
- Low/high sensitivity around the entered total: Enter the quantity described by the label.
- Explicit tax allowance: Enter the quantity described by the label.
Reproducible worked example
The following is a fictional arithmetic example, not a local quote, a verified claimant or a market forecast. Its values are included so the formula can be checked. The form does not automatically confirm any verification checkbox.
| Input | Example value |
|---|---|
| Number of windows | 10 |
| Average area of one window (m²) | 1.5 |
| Existing window U-value (W/(m²·K)) | 2.8 |
| Proposed window U-value (W/(m²·K)) | 1.3 |
| Local heating degree days (°C·days) | 2,500 |
| Seasonal heating efficiency | 90 |
| Heating energy cost per input kWh | 0.1 |
| Material price per window | 0 |
| Total labor allowance | 0 |
| Delivery allowance | 0 |
| Other fixed costs | 0 |
| Material/labor multiplier | 1 |
| Low/high sensitivity around the entered total | 15 |
| Explicit tax allowance | 0 |
| Output | Calculated result |
|---|---|
| Total window area | 15 m² |
| Modeled heating input saved, kWh | 1,500 |
| Modeled annual heating saving | GBP 150 |
| Material cost | GBP 0 |
| Labor allowance | GBP 0 |
| Delivery allowance | GBP 0 |
| Other specified costs | GBP 0 |
| Entered tax allowance | GBP 0 |
| Low cost scenario | GBP 0 |
| Middle cost scenario | GBP 0 |
| High cost scenario | GBP 0 |
Interpreting a changed assumption
Start with one complete scenario and change only one input when diagnosing a difference. Recheck units, the period of every rate, whether an amount is recurring or one-off, and whether a cost is already included elsewhere. A larger calculated benefit is not evidence that an assumption is more likely to occur.
Limitations
- Quantities and user-entered costs are planning estimates. Drawings, manufacturer instructions, local permits and qualified site-specific design take precedence.
- Material is unpriced. A zero material price is not evidence that the material is free.
- Prices and the tax allowance are user-entered planning assumptions, not a verified local quote. Low/high outputs are sensitivity scenarios, not market confidence limits.
- The multiplier applies only to material and labor; delivery and fixed costs are not multiplied.
- Whole-window U-values must be comparable. Solar gain, air leakage, shading, rebates and maintenance are not inferred. Negative savings means the entered replacement conducts more heat.
Frequently asked questions
What does this Window Replacement result represent?
Combine a quoted per-window replacement price with a conductive heat-loss comparison using whole-window U-values. It applies the displayed formula to your inputs; it does not infer omitted facts.
Are the starting amounts verified United Kingdom prices or legal rules?
No. Numeric defaults are illustrative unless an input or dated reference explicitly identifies a researched component. Currency is GBP; local quotes, eligibility and unsupported rules must be entered and verified separately.
What must I check before relying on the result?
Quantities and user-entered costs are planning estimates. Drawings, manufacturer instructions, local permits and qualified site-specific design take precedence.
Can I compare or share different assumptions?
Change one assumption at a time and use the browser-local project or share controls. A shared URL contains the encoded input values; do not include information you do not wish to disclose.
Sources and reference scope
- NIST unit conversion guidance — retrieved 2026-09-24. Unit definitions; geometry and financial arithmetic in these worksheets are explicitly stated.
- DOE insulation overview — retrieved 2026-09-24. Insulation and thermal resistance concepts; not structural or code approval.
These references support only their stated scope. They are not evidence that every local rule or price needed by the master expansion has been loaded.