SPF Blend Estimate Calculator

Work out spf blend estimate instantly with clear inputs, formula shown and shareable results.

About SPF Blend Estimate Calculator

UV filter contributions are roughly additive in an idealised film, but real films are uneven so a uniformity factor discounts the sum. SPF has diminishing returns: SPF 15 blocks 93.3% of UVB, SPF 30 blocks 96.7% and SPF 50 blocks 98%. Doubling SPF does not halve exposure risk in practice.

Formulas

Additive contributions

Estimated SPF = 1 + (sum of filter contributions - 1) x film uniformity factor

Protection

UVB blocked % = (1 - 1/SPF) x 100

Worked example

With the default values below, SPF Blend Estimate Calculator returns Estimated SPF of 22.250. Change any input above and every figure here is recalculated in your browser.

Inputs used

Filter A Contributionminimum 0 SPF units: 8 SPF units
Filter B Contributionminimum 0 SPF units: 12 SPF units
Filter C Contributionminimum 0 SPF units: 6 SPF units
Film Uniformity Factoraccepts 1–100 %: 85 %

Results

Estimated SPF: 22.250
Additive Sum Of Contributions: 26.00
UVB Radiation Blocked: 95.5056%
Indicative Label Category: 15

Step by step

1Values used
2Additive contributions
3Protection
4Estimated SPF= 22.250
5Additive Sum Of Contributions= 26.00
6UVB Radiation Blocked= 95.5056
7Indicative Label Category= 15

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
SPF and broad-spectrum claims must be substantiated by approved test methods. Calculated estimates cannot be used to support a label claim.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

SPF and broad-spectrum claims must be substantiated by approved test methods. Calculated estimates cannot be used to support a label claim.

Frequently Asked Questions

Can SPF be calculated rather than tested?

No. In vitro and in silico estimates guide formulation, but a claimed SPF must be established by an approved in vivo or validated in vitro method. Calculated values have no regulatory standing.

Why does application amount matter so much?

SPF is measured at 2 mg/cm2, far more than consumers typically apply. Applying half that amount gives roughly the square root of the labelled SPF, so an SPF 30 behaves like SPF 5-6.