Methodology
How Swap Priority works
Last updated 25 September 2026
Swap Priority is a 0–100 score that answers one question: which plastic should you replace first? It rewards swaps that cut the most direct, frequent contact with plastic, avoid the most plastic, and are easy and affordable — then adjusts for how strong the evidence is.
The five inputs
| Input | Scale | What it measures | Weight |
|---|---|---|---|
| Exposure | 0–5 | How directly the plastic touches food, drink, skin or heat. Heating fatty or acidic food in plastic scores 5. | 30% |
| Frequency | 0–5 | How often a typical household uses it. Daily is 5; weekly is 3. | 20% |
| Plastic avoided | 0–5 | Grams of plastic avoided per household per year, log-scaled. 2 kg or more scores 5. | 20% |
| Ease | 0–5 | How simple the switch is: availability, learning curve, fit with daily life. | 15% |
| Cost | 0–5 | Cost over a year compared with the plastic version. 5 saves money; 0 costs much more. | 15% |
The formula
raw = (0.30 × Exposure + 0.20 × Frequency + 0.20 × Mass + 0.15 × Ease + 0.15 × Cost) ÷ 5
evidence = (2 × Exposure evidence + Environmental evidence) ÷ 12
score = 100 × raw × (0.6 + 0.4 × evidence)
Mass is 5 × log10(1 + grams) ÷ log10(2001), capped at 5. We use a log scale so a swap saving 2 kg does not swamp one saving 200 g that touches your food every day.
The evidence adjustment uses the first two Evidence Dials. Exposure evidence counts double because exposure carries the largest weight. Weak evidence can pull a score down to 60% of its raw value, never lower.
Bands
| Score | Band | What it means |
|---|---|---|
| 75–100 | Swap first | Direct, frequent contact with strong evidence. Start here. |
| 60–74 | High priority | Clear benefit; do these next. |
| 45–59 | Worth doing | Real but smaller gains. |
| 0–44 | Lower priority | Fine to do, but not where the biggest wins are. |
Worked example
Reheating food in plastic tubs → glass containers. Exposure 5, frequency 4, 600 g a year, ease 4, cost 3. Exposure evidence 3, environmental evidence 3.
- Mass score: 5 × log10(601) ÷ log10(2001) = 4.21
- Raw: (1.50 + 0.80 + 0.84 + 0.60 + 0.45) ÷ 5 = 0.838
- Evidence: (6 + 3) ÷ 12 = 0.75, so the multiplier is 0.9
- Score: 100 × 0.838 × 0.9 = 75 — Swap first
Plastic carrier bags → cotton tote. Exposure 0, frequency 3, 400 g, ease 5, cost 4; both evidence dials 1. Score: 38 — Lower priority. The bag never touches your food, and a tote needs many uses to pay back its footprint.
Your personal score
The Home Plastic Audit re-runs this formula with your household's answers. Your frequency replaces the typical one, and plastic avoided scales with it (between a quarter and double the typical amount). Two households can get different top three swaps, and that is the point.
What Swap Priority is not
It is not a health-risk score and it is not a product rating. Products are judged by their materials and by Claim verification. Commercial factors — affiliate commission, brand relationships — are never inputs. See our independence rules.