What the number actually is
Response rate is responses divided by pieces mailed. Mail 500, get 10 responses, that is 2%.
The ambiguity is in "response". Is a scan a response? A phone call? Only a booked appointment? Different definitions produce wildly different rates for the same campaign, which is exactly why published benchmarks are hard to compare against.
Define it once, write the definition down, and use the same one every time. Your own consistency matters far more than matching somebody else's methodology.
Why benchmarks mislead
Direct mail response rates are commonly cited in a range from well under one percent to several percent. Both ends of that are true, for different campaigns.
The biggest driver is who received it. Mail to your own lapsed customers routinely responds many times better than mail to a purchased list of strangers. Comparing your reactivation drop to a general benchmark tells you nothing.
The second is what counts as a response. A campaign counting scans will always look better than one counting completed bookings. When you see a benchmark, you usually cannot tell which was measured.
The number that actually decides
Cost per acquisition is the metric that answers the question you care about, which is whether to do it again.
A 0.5% response on a 2,000-piece drop at $1.19 is 10 customers for $2,380 — $238 each. Whether that is good depends entirely on what a customer is worth to you over the relationship. For a dental practice it is cheap. For a coffee shop it is ruinous.
Meanwhile a 4% response can lose money if the offer discounted the sale below its cost. High response rates on unprofitable offers are the most flattering way to lose money in marketing.
Use rate to compare yourself to yourself
Response rate is genuinely useful in one place: comparing your own drops.
Same list, two different offers — the rate tells you which offer worked. Same offer, two different segments — the rate tells you which audience is more responsive. That is a controlled comparison, and it is actionable.
Keep a simple record of every drop: date, list description, size, offer, and responses. After three or four campaigns you will have something worth more than any published benchmark, because it is about your business.
What moves it most
In rough order: who you mailed, what you offered, when it arrived, and only then how the card looked.
Switching from a cold list to your own lapsed customers can change the rate by a multiple. Sharpening a vague offer into a specific one with a deadline can change it substantially. Arriving in the right week for a seasonal service matters more than most people expect.
Design matters, but it is the smallest of the four, and it is the one most businesses spend the most time on.