Opening weekend gets the headlines. The number that tells you whether a film is actually working is what happens the following weekend, and it is reported far less prominently.
What the drop measures
Opening weekend is a measure of marketing. It reflects awareness, anticipation and the size of the campaign, and it happens before anybody has told anybody else whether the film is any good.
The second weekend is the first measure of the film itself. By then, the people who saw it have spoken to their friends, reviews have circulated, and the audience arriving has some information.
So the percentage decline between the two is a reasonably direct read on word of mouth.
A shallow drop means people are recommending it. A steep drop means the marketing worked and the film did not.
The rough scale
Conventions vary by market and genre, and the general pattern holds.
A drop of under half is considered strong and indicates genuine enthusiasm.
Something around half to sixty percent is normal for a wide release.
Above about two thirds indicates a problem, and very steep drops beyond that are the signature of a film that front-loaded its entire audience into the opening.
What distorts it
Several things, which is why the number needs context.
Genre. Horror front-loads heavily, because the audience is young, enthusiastic and turns out immediately, and steep drops are normal rather than a failure signal.
Family films hold much better, because attendance is spread across weekends by parents' schedules rather than by enthusiasm.
Competition. A major release the following week takes screens and attention regardless of quality.
Holidays, which compress or extend the effect substantially.
And opening size itself, since a film that opened enormously has fewer people left to convert.
The legs concept
The related and more useful measure over a full run.
A multiplier is calculated by dividing total takings by the opening weekend. A film that takes three or four times its opening has genuine legs; one that takes barely twice has not.
Historically, multipliers were much higher across the board, because films opened on fewer screens and expanded as word spread.
The shift to enormous simultaneous openings has compressed multipliers everywhere, which means comparing across eras requires care and that modern films with strong legs are unusual and significant.
Why the industry watches it so closely
Because of how the money is split.
Exhibitors and distributors share takings on terms that shift over the run, with the distributor taking a larger share early and the exhibitor's share rising in later weeks.
Which means a front-loaded film is worth relatively more to the distributor and a film with legs is worth more to the cinema, and the two parties are watching the same number with different interests.
It also determines screen allocation. A film dropping steeply loses screens quickly, which accelerates the decline in a way that becomes self-reinforcing.
What it does not tell you
Worth being clear about the limits.
It says nothing about profitability, since production and marketing costs are not public and reported figures are gross rather than net.
It says nothing about a film's eventual reputation. Several films with catastrophic drops have become durable favourites, and several with excellent legs are now forgotten.
And it is increasingly incomplete as a picture of performance, since a substantial share of a film's audience now arrives later through streaming and does not appear in any box office figure at all.
How to read the reporting
Ignore the opening weekend headline, which measures the campaign.
Look at the second weekend percentage, adjusted for what genre would be expected.
Look at the multiplier once the run is over, which is the honest summary.
And treat any claim about profitability with suspicion, since the numbers required to assess it are not published and the ones that are published are chosen by people with an interest in how they read.
The tracking that precedes it
Worth knowing about because it explains why studios seem to know outcomes in advance.
Audience tracking surveys run in the weeks before release, measuring awareness, interest and intent to see, broken down by demographic.
These are reasonably predictive of opening weekend and considerably less predictive of what happens afterwards, because they measure the effect of marketing rather than the quality of the film.
Which is why a studio can be confident about an opening and genuinely uncertain about the run, and why the tone of coverage frequently shifts sharply in the second week.
Exit polling
The other instrument, and the more informative one.
Audiences are surveyed as they leave, producing a grade that has historically been a good predictor of legs, because it measures whether people will recommend it.
A film with a strong opening and a poor exit grade is generally understood within the industry to be finished, days before the second weekend confirms it.
The grades are widely reported and are one of the more useful public numbers, partly because the scale is compressed — audiences self-select into films they expect to like, so anything below a middling grade indicates real dissatisfaction rather than mild disappointment.