So your newspaper is thinking about putting up a metered paywall. What kind of results can you expect?

The answer will depend on a lot of factors — the makeup of your audience, how tight or loose you set the meter, what you’re charging. But from looking at other newspapers’ results — more than 500 have a paywall in the U.S. alone — you can start to estimate.

Here’s one data point, from paywall service provider Press+, which runs hundreds of those sites. Of a site’s monthly uniques, about 3 to 4 percent will hit the paywall — that is, they’ll consume enough content to be told they have to pay to read more. And about 0.5 percent of your monthly uniques will hit the wall and actually pay.

That’s from this article by Jeff Hartley, vice president of consumer revenue for Morris Publishing Group, at the Newspaper Association of America site.

With the help of our digital subscription management partner, Press+, we know what percentage of our unique visitors is encountering a stop light box (SR) and how many, once stopped, are converting to a subscription (PSCR). Because Press+ has so many affiliates, 450 and counting, they’ve established performance benchmarks for both the SR and the PSCR. These indicators are crucial to knowing how effective you are in engaging readers and converting them to paid sales (Note: these metrics can be set these up on your own using Google Analytics, Omniture, etc.)

Based on data from Press+, the average publication has a stop rate (the number of unique visitors that see a stop light box) of 3%-4%, but high performers stop between 5%-10% of unique visitors. A PSCR rate (the number of unique visitors that are stopped and then purchase) of 0.5% is considered average, while high performers convert 1%-2% of stops into paying subscribers.

(You may notice that that last sentence is a little screwy — the first half says it’s measuring the share of unique visitors, while the second half measures share of stops. I’m pretty sure the second half of that last sentence is misworded — it should be uniques. Otherwise we’d be talking truly tiny numbers — 0.03 × 0.005 — which might buy you a couple cups of coffee, but not much more.)

So, back of the envelope math: Your site gets 1 million monthly unique visitors. A typical metered paywall would be invisible to the vast majority of them — they won’t read enough stories to hit it. On average, you might expect 30,000 to 40,000 to hit the paywall — but that could be as high as 50,000 to 100,000. Of those stopped, you’ll probably get 5,000 to pay up — if you’re a top performer, maybe 10,000 or 20,000.

Still, even within Press+ customers, there’s a wide range of outcomes; as Ken Doctor reported for us last November, the best Press+ paywalls far outperform the worst. The details matter:

While each publisher gets its own data, Press+ gets it all. Out of that, you get one of the major selling points Press+ uses with its clients: “We’ll share best practices with you.” Press+ has measured the differential between its highest- and lowest-performing sites at 10-to-1, and that would be an impressive difference-maker in revenue return. Get the metering, marketing, messaging, and more right, Press+ says, and you’ll more than make up the revenue share (~20 percent) you pay us.