If you stopped acquiring customers today, how long would it take for half your existing-customer revenue to disappear?
Three months?
Nine months?
Two years?
Most CEOs couldn't answer that question.
But I think it tells you something extremely important about the quality of an ecommerce business.
I call it:
Customer Base Half-Life
The idea comes from radioactive decay.
A substance's half-life tells you how long it takes for half of it to disappear.
Your customer base has something surprisingly similar: Stop adding new customers and revenue from the customers you already have will gradually decay.
Some customers reorder quickly.
Some return occasionally.
Some disappear.
The time it takes to get there is your Customer Base Half-Life.
This is different from repeat purchase rate
Repeat purchase rate tells you how many customers purchase again. But it doesn't capture the full shape of the decay.
The half-life forces you to look at the entire customer revenue curve.
It is also different from LTV, which asks:
How much is a customer worth?
Half-life asks:
How durable is the revenue engine created by the customer base?
Those are related, but not identical.
A high-ticket business could produce respectable LTV from a relatively short customer relationship.
A lower-ticket replenishment business could have an extraordinarily long customer half-life.
And two segments with similar projected LTV can produce that value across very different periods.
One metric is about magnitude. The other is about persistence.
Here's how I'd calculate it
If we acquired no additional customers from this point forward, how would revenue from this exact population decay over time?
Using RetentionX's revenue cohorts, you can create the curve and additionally analyze its causation:
• purchase cadence
• replenishment patterns
• time since last order
• customer tenure
• order frequency
• product mix
• observed survival curves
You are not assuming marketing stops. Existing customers can still receive normal lifecycle communication.
You're simply removing new customers from the equation.
Then plot the expected monthly revenue generated only by today's customer base.
The moment that revenue falls to 50% of its starting level gives you the half-life.
For some brands that might be 4.7 months for others 21.3 months.
Half-life also creates a much better retention target
“Improve retention” is vague.
“Extend customer half-life from 9 months to 12 months” is much more interesting.
It forces the business to ask what actually makes the customer base durable.
Maybe it is:
• getting more customers into order #2
• creating a habitual replenishment product
• building better category expansion
• avoiding over-discounting
• improving product quality
• making subscription cadence work better
• preventing high-value customers from becoming dormant
Every one of those actions changes the shape of the decay curve.
And then there is the really uncomfortable calculation
Once you know the half-life, you can estimate how much acquisition is simply required to keep replacing the customer revenue that naturally decays.
Suppose your existing base loses $1.5M of monthly revenue capacity as customers disappear.
Before acquisition creates one dollar of net growth, it has to refill that hole.
That creates a useful distinction:
Replacement acquisition
versus
Expansion acquisition
A company might spend $20M a year “on growth.”
But if $13M is economically required just to offset customer-base decay, only part of that spend is actually expanding the business.
The rest is keeping the machine at its current size.
A longer half-life changes the economics of almost everything
If you increase customer half-life:
CAC becomes easier to recover.
Future revenue becomes more predictable.
Growth requires less replacement acquisition.
Marketing disruptions become less dangerous.
The company has more flexibility around capital.
And every customer acquired today keeps contributing for longer before another customer needs to replace them.
That is why I think half-life may be one of the cleanest ways to describe the durability of a customer base.
The takeaway
Revenue tells you how large the business is.
Growth tells you how quickly it is expanding.
LTV tells you how much customers are worth.
But none of those answers one very simple question:
If you stopped feeding new customers into the machine, how quickly would the existing business disappear?
Because the strongest ecommerce businesses aren't just good at acquiring customers.
They build customer bases that take a very long time to decay.
What is your Customer Base Half-Life?
Listen on Spotify
