Witness translates dollars into stories. To do that we lean on public cost-effectiveness research and apply simple per-dollar formulas. This page is the full reference so you can sanity-check every figure.

Sources

Cost-per-unit and cost-per-life figures: GiveWell top-charity pages (2022–2024 averages).
Displayed impact range: GiveWell, “How much does it cost to save a life?” (published April 2024).
Life-expectancy estimates by country and gender: UN World Population Prospects, latest revision.
UK doctor career comparison: the 80,000 Hours estimate of roughly 100 combined extra healthy life-years, equivalent to about 3 lives saved, over an average UK doctor's career.

Per-charity unit costs

For each gift, Witness divides the amount by the program's cost-per-unit to get the number of units unlocked:

Malaria Consortium (SMC): ~$7 per child protected (GiveWell overview, September 2025) · ~$4,000 per life saved (funding directed in 2022–2024).
Against Malaria Foundation (nets): ~$6 per net (GiveWell overview, September 2025) · ~$5,500 per life saved (funding directed in 2022–2024).
Helen Keller Intl (vitamin A): ~$2 per child supplemented for one year (GiveWell overview, September 2025) · ~$3,500 per life saved (funding directed in 2022–2024).
New Incentives (vaccines): ~$146 per infant vaccinated (GiveWell overview, September 2025) · ~$4,500 per life saved (funding directed in 2022–2024).

Lives saved (estimate)

lower_estimate = eligible_donation_amount / 5,500
upper_estimate = eligible_donation_amount / 3,000

In April 2024, GiveWell reported that its top-charity models over the preceding several years typically estimated $3,000–$5,500 per life saved. Witness applies these historical USD bounds to gifts with a life-saving impact estimate; other charities do not contribute to this figure. For example, $1,200 gives 0.2–0.4 estimated lives saved. Positive estimates entirely below 0.1 are shown as “< 0.1”; an empty donation log shows zero.

This is an illustrative range, not a statistical confidence interval or a guarantee. A particular gift may have more or less impact depending on geography, timing, program execution and modelling assumptions. Concrete per-program units stay unchanged. Tiers and awards retain their existing per-program estimates.

GiveWell expects the cost of saving a life to rise over time.

Healthy years gained

Both endpoints of the lives-saved range are multiplied by 100 / 3, using the doctor comparison's approximate 100 healthy life-years per 3 lives saved. This is a simplified conversion, not a separate GiveWell estimate of healthy life-years. Doctor career equivalents divide both endpoints by 3; percentage comparisons multiply by 100.

Lifetime projection

Uses your age, country/gender life expectancy, and the larger of stated annual giving or annualised recurring gifts:

years_remaining = max(0, life_expectancy − age)
projected_annual = max(annual_giving, recurring_annual)
future_giving = projected_annual × years_remaining
lifetime_lower = logged_lower + future_giving / 5,500
lifetime_upper = logged_upper + future_giving / 3,000

The sentence and lifetime statistics show the same range, including eligible logged gifts plus future giving. With no logged gifts, this remains a what-if projection based on your annual giving, not impact already achieved. Future giving assumes top-charity effectiveness at the historical $3,000–$5,500 range; no cost inflation is modelled, so the projection may overstate future impact as costs rise.

Currency display

Switching the display currency in your profile changes the symbol only — historical amounts are not converted at live FX rates. This avoids implying spurious precision.

What this is not

These calculations are illustrative. They are not a substitute for a charity's own impact report, GiveWell's full cost-effectiveness analysis, or professional financial advice. Treat the numbers as order-of-magnitude estimates that help make giving emotionally tangible.