Interchange and Incremental Spend: The Maths Behind Offers

Finance teams do not approve offer programmes because they sound engaging. They approve them because the numbers work. Yet the maths behind card-linked offers is often presented either too vaguely ("offers drive engagement") or too optimistically ("every redemption is new revenue").

This guide walks through a simple, honest model. Every number below is hypothetical, chosen only to make the arithmetic easy to follow. Replace them with your own portfolio data, your market's interchange rates and your actual campaign results.

The four quantities you need

To estimate the economics of a campaign, you need four things:

1. Incremental spend: the extra spend on your cards caused by the campaign, measured against a control group.
2. Revenue rate: what the bank earns per unit of card spend. For most issuers this is mainly interchange, plus any other income linked to usage.
3. Funding cost: what the bank pays towards rewards. For merchant-funded offers this may be zero; for bank-funded or co-funded offers it is the bank's share.
4. Running cost: the platform, marketing and operational costs of the campaign.

The core equation is simple:

Campaign return = (incremental spend × revenue rate) − funding cost − running cost

The hard part is not the equation. It is estimating incremental spend honestly.

Step 1: separate total spend from incremental spend

Suppose, hypothetically, a bank runs a four-week dining campaign. Customers who redeem the offer spend a total of 1,000,000 units of currency at participating restaurants.

It is tempting to treat that whole amount as the campaign's result. It is not. Many of those customers would have eaten out anyway, and some would have used your card.

The bank therefore compares the exposed group with a random holdout group that did not see the offer. Suppose that comparison shows the exposed group spent 20% more on dining with the bank's cards than the holdout, relative to group size. In our hypothetical example, this translates to 150,000 units of incremental spend across the campaign.

Note the gap: 1,000,000 in redeemed spend, but 150,000 in incremental spend. That gap is normal, and ignoring it is the most common mistake in offer reporting.

Step 2: apply the revenue rate

Next, estimate what the bank earns on that incremental spend. Suppose the blended revenue rate, mostly interchange, is 1.5% for the cards and merchant categories involved. In many markets it will be lower, especially where interchange is capped.

Incremental revenue = 150,000 × 1.5% = 2,250

On its own, that looks small. That is why the next steps matter.

Step 3: count the funding cost correctly

The bank's funding cost depends on who pays for the reward.

• Merchant-funded: the merchant covers the discount. The bank's funding cost is zero.
• Bank-funded: the bank pays the full reward on every redemption, including redemptions by customers who would have come anyway.
• Co-funded: the cost is shared.

Suppose the rewards were worth 5% of redeemed spend. If the bank funded them fully, the cost would be 5% × 1,000,000 = 50,000. Against 2,250 of incremental revenue, the campaign loses heavily.

If the offer is merchant-funded, the bank's funding cost is zero, and the picture changes completely.

This is the single most important lesson in offer economics: bank-funded rewards are paid on all redemptions, but only earn on incremental ones.

Step 4: add running costs

Suppose platform and marketing costs allocated to the campaign are 1,500. In the merchant-funded case:

Campaign return = 2,250 − 0 − 1,500 = 750

Positive, but modest. If that were the whole story, many banks would conclude offers are barely worth it. It is not the whole story.

Step 5: look beyond the campaign window

Short-term interchange understates the value of offers because the most important effects continue after the campaign:

• Habit persistence: some customers keep using the card for dining after the offer ends.
• Reactivation: dormant cards that were used again may stay active.
• Retention: engaged customers are less likely to close accounts.
• Wider relationship: more frequent card use tends to go with salary deposits and product uptake.

Suppose, hypothetically, the holdout comparison shows that half of the incremental dining spend continues for the next three months. That adds further incremental spend and revenue at no additional funding cost. The same logic applies, with caution, to retention and relationship value. These should be estimated from your own data, not assumed.

A simple summary table

The table does not say bank funding is always wrong. It says bank funding must be targeted where incrementality is highest, such as new-to-category customers, dormant cards or competitive switching, and sized carefully.

Improving the equation

Each term in the equation can be improved:

• Raise incrementality by targeting customers who do not already buy in that category with your card.
• Lower funding cost by using merchant-funded offers as the base and bank funding only for strategic segments.
• Cap exposure with budgets, redemption limits and time windows.
• Lower running costs with automated campaign setup, targeting and reporting.
• Extend the effect by designing offers that build habits, not just one-off visits.

The takeaway

If you want to run this model with your own inputs, the business case calculator does the arithmetic and shows every assumption.

Honest offer maths starts with incremental spend, not total redemptions. Interchange on incremental spend is usually modest, so funding strategy decides whether a campaign creates or destroys value. Merchant-funded offers, precise targeting and control groups turn card-linked offers from a cost centre into a measurable growth lever.