When will we know who won Philly’s mayoral primary election? Depends on how close the votes are.

So many people want to be Philly’s next mayor that it could take a long time to find out who actually wins.

That’s because it’s a lot harder to be certain who won if the margins are slim, and with five major candidates splitting the votes in the city’s Democratic primary, there’s a real possibility of even a small number of votes changing the result.

» READ MORE: Philadelphia’s primary election is next Tuesday. Here’s your no-nonsense guide to the races for mayor, Council, and row offices.

Unlike in some recent statewide elections, most votes will be counted fairly quickly, so a race call on election night is certainly still very possible. But if the race is as close as some observers expect, a fast vote count may not be enough to determine a clear winner by early Wednesday morning. And if that’s the case, it could take days or even weeks for clarity as the final votes are counted, especially if things end up in court.

Whether we find out the winner in a matter of hours or days all depends on the margins.

“If results are close, results are going to be close, and it’s going to be a while to know,” said Nick Custodio, deputy to Philadelphia elections chief Lisa Deeley and spokesperson for the elections office. “If races aren’t called by Wednesday morning, people are in for a long haul.”

Here’s what to know about the vote count this election and what to expect.

The official answer to when we know the winner hasn’t changed: The vote is official when it’s certified, which happens about three weeks after election day unless there’s a recount or protracted litigation.

Realistically, the answer is usually clear long before certification, because news organizations “call” a race, informally declaring the victor once the results are clear. The Inquirer relies on the Associated Press for those calls, and the AP has a long track record of accuracy at the national, state, and local levels.

That accuracy takes a lot of work. As results stream in, sophisticated statistical models compare the numbers to past voting patterns; analysts add on-the-ground knowledge of the political landscape to watch for the moment when it’s clear a candidate has won.

Getting to that moment requires enough votes to be counted that the patterns are clear and the margins strong enough that they won’t flip as more votes come in.

Remember waiting for days in November 2020 for Pennsylvania to be called? That was because the vote count took a long time — and the numbers were very close for a long time. When it took more than two weeks to declare a victor in last year’s GOP Senate primary, it was because the margins were so tight they triggered an automatic recount. Last November, it took more than a week to know who won the state House of Representatives because totals in two suburban Philadelphia districts came down to just dozens of votes each.

The vast majority of votes should be counted within a few hours of polls closing.

It will still take days to fully count every last vote, but nearly all the results will be reported very quickly. That’s in large part because the number of mail ballots in this election is relatively low.

Turnout in general will be lower this election than in a federal election, and Philly voters still vote in person more than they do by mail. (In recent elections, mail ballots have made up a quarter to a third of the votes cast.) In-person results are tracked by voting machines as they’re used throughout the day, so reporting the results is simply a matter of uploading the results at the end of the night. That process is usually done by about 1 or 2 a.m.

Mail ballots are the ones that usually take longer to count. In this election, there will be fewer than 100,000 mail ballots to count, as more than 96,000 Philly voters successfully requested mail ballots, and not every voter will return a ballot. City elections officials expect to have most of them counted that night, with the bulk of the remainder counted on Wednesday.

Pennsylvania elections officials have become much more efficient with counting mail ballots, including by purchasing more equipment; increasing staffing; improving their procedures; and generally becoming faster and better through experience. Most counties, including Philly, will also be required to count mail ballots around the clock until they’re done.

With the votes counted quickly, the real question is about the margins.

That’s where things get messy.

There are five major candidates for mayor splitting the vote, with all of them clumped together within striking distance of each other. That means each candidate will receive a much lower number of votes than in a smaller field and one could win with, say, just 25% of the vote. As a result, the winning vote total will likely be much, much smaller than in, say, a presidential race or U.S. Senate contest.

And that creates a challenge for calling the race: When a small number of votes can change the outcome, election officials need a lot more votes to be able to declare a winner.

Election workers begin counting mail ballots at 7 a.m. election day, and those vote totals are the first to be released shortly after polls close at 8 p.m. Those results should reflect most of the mail vote — every mail ballot received by a day or two before election day, when the poll book data is updated.

After that large batch of mail ballots is reported, the city begins the hours-long process of receiving the in-person results. By about 1 or 2 a.m., more than 90% of the vote will likely have been counted and reported.

On Wednesday morning, workers will work through the remaining mail ballots that came in in the final days of the race, and continue to track down any remaining voting machine results that haven’t been returned.

At that point, by sometime Wednesday afternoon, nearly all votes will have been reported. The only votes remaining are a handful of overseas and military ballots that can continue trickling in through Tuesday; provisional ballots, which can number in the thousands, but many of which will be rejected; mail ballots that have some issue and require the Board of Elections to decide whether to count or reject; and any in-person voting machine results that haven’t yet been tracked down, such as when a poll worker locks the machine in a closet at the end of the night instead of returning the results cartridge.

So the vast majority of votes will be in by the end of the night. If race results aren’t announced by then, the next likeliest scenario is Wednesday morning, as the remaining mail ballots are tallied.

After that, if things are still too close for a winner to be declared, we could be in for a long and bumpy ride.