Quick answer
Visa divides chargebacks this month by transactions this month. Mastercard divides chargebacks this month by transactions last month. Your acquirer may use a different rolling window. All three numbers can be different — and all three matter. Calculate each one separately and manage to the lowest threshold.
Why the formula differs by network
Visa and Mastercard designed their programs independently. Mastercard's lagging denominator (prior month transactions) was intended to smooth out seasonal volume spikes. The practical effect: if your volume dropped significantly last month, your denominator is smaller and your ratio looks worse this month — even if the raw chargeback count stayed the same.
This is not a technicality — merchants who do not model both formulas have been surprised by Mastercard ECM enrollment because their Visa ratio looked fine while their Mastercard ratio crossed the threshold silently.
The three formulas you need to track
Visa (VAMP)
Chargebacks in current month ÷ transactions in current month
Threshold: 0.9% warning / 1.8% termination
VAMP also includes an enumeration component separate from the ratio. Both must be monitored.
Mastercard (ECM)
Chargebacks in current month ÷ transactions in prior month
Threshold: 1.5% (ECM) / 3.0% (HECM) — AND minimum count thresholds apply
Mastercard's lagging denominator means your ratio today reflects business from 30+ days ago.
Most acquirers
Chargebacks in rolling 30 days ÷ transactions in rolling 30 days
Threshold: Varies by acquirer — typically 1.0% is their internal warning
Acquirers often use a tighter internal threshold than the card network programs.
A worked example
Scenario: In May, you processed 10,000 transactions. In June, you received 120 chargebacks and processed 8,000 transactions (volume dropped due to seasonal slowdown).
Visa (current month denominator): 120 ÷ 8,000 = 1.50% — ABOVE warning threshold
Mastercard (prior month denominator): 120 ÷ 10,000 = 1.20% — BELOW ECM threshold of 1.5%
Acquirer (rolling 30 days, same counts): May vary — depends on their window definition
The same 120 chargebacks produce a Visa warning-level ratio and a Mastercard ratio that looks fine — purely because of the formula difference. If you only tracked one, you might miss the Visa concern entirely.
Set your internal warning earlier
The published thresholds are enforcement lines — not targets. Your internal warning should trigger well before the threshold: 0.7% for Visa, 1.0% for Mastercard, and whatever floor your acquirer uses. That buffer gives you 30–60 days to act before a program enrollment becomes possible.
For the full threshold context and what happens when you breach them, see Visa and Mastercard chargeback thresholds explained. HighRiskIntel calculates both Visa and Mastercard ratios from your transaction data automatically and alerts you when either approaches your configured warning level.