Are You Actually On Pace for a PB?

The lead is the easy bit

Your PB read 15:00 at this split. You're at 14:30. You're thirty seconds up.

That part is subtraction, and it has a property worth holding on to: a second saved in the first segment is worth exactly as much as a second saved in the last. Seconds don't compound. Percentages do, which is why thinking in percentages here misleads people.

The hard bit is the next question — what does it finish in?

There are two honest answers

Not an optimistic one and a cautious one. Two genuinely different assumptions, each valid in different circumstances.

Flat — the rest of the run matches your PB exactly. You keep the thirty seconds and gain nothing more. On a 1:00:00 PB that's a 59:30 finish.

Scaled — you carry on outperforming at the same rate you have so far, so the lead grows in proportion. That's a 58:00 finish.

Same split, same lead, ninety seconds apart.

The spread is bigger than the lead

That's the part worth internalising, and it has a clean formula:

spread = your lead × (1 ÷ fraction of the run done − 1)

How far through	Spread, as a multiple of your lead
25%	3×
50%	1×
75%	0.33×
90%	~0.11×

A quarter of the way in, a thirty-second lead spans ninety seconds of possible finishes — a range three times the size of the thing you're excited about.

This is the arithmetic under something every runner already feels: an early split tells you almost nothing, and a late split tells you a lot. It's not superstition about jinxing a run. The information genuinely isn't there yet.

So which number is real?

One question decides it: was the time repeatable?

Execution scales. A lead built from cleaner movement, tighter routing, fewer small mistakes — those keep happening for the rest of the run. The scaled projection is the fair one.

Luck doesn't. A lead from one good drop, a favourable spawn, or a skip that either works or doesn't is a one-off. It's already banked and it won't repeat. The flat projection is honest.

Most real leads are a mix, which is exactly why the tool gives you both instead of averaging them into a single confident-looking number. Hold them as bounds. The blend is your judgement, and it's a judgement you can actually make — you were there when the time was saved.

The number to act on

Pace the rest needs, against your PB is the actionable output, because it's the one that translates into how to play.

Chasing 59:00 from thirty seconds up at the 15:00 split, it reads 98.9%. The rest of the run has to be about one percent faster than your PB managed.

That's the sting. A comfortable-looking lead can still require the remainder to beat your best rather than match it — which is a completely different instruction to yourself than "hold on". Under 100% means push. Over 100% means you have slack and can afford the safe strats.

If it comes out negative, the goal already went by at this split.

What it won't tell you

Whether to reset. That depends on how much time is realistically left to gain, how much of your evening you'll spend on one attempt, and whether finishing a non-PB run has value to you. For a lot of runners it does — the practice is in the back half, and resetting means never practising it.

What's actually achievable. Your sum of best segments is a better guide than any projection from a single split, because it's built from times you've genuinely hit rather than an assumption about the future. If your splits software tracks it, that's the number to plan around between sessions.

This tool works from what you can read off a timer mid-run. That's its job, and it's worth being clear about where that job ends.

One practical note

Everything is in seconds, because a timer reading of 12:34 is two numbers and a formula needs one. Multiply the minutes by 60 and add the seconds — 12:34 is 754, 1:02:30 is 3750.

The printable carries a conversion table, which is the point of having it open next to you rather than doing arithmetic while a run is live.