How much chalk do you actually need for a session?

This depends more on how often you're re-chalking than on any fixed amount, but there's a reasonable way to estimate it.

Think in chalk-ups, not grams

A single light coat, applied correctly, uses a small amount, closer to a pinch than a handful. What actually determines how much chalk you go through in a session is how many times you reapply, not how much you use per application. A climber re-chalking every attempt on a long bouldering session will go through noticeably more chalk than someone climbing a couple of longer routes with fewer stops.

A rough way to estimate

If you're climbing for an hour and reapplying every few attempts, you're likely using somewhere in the range of a few grams total, not a significant fraction of a bag. A 150g container is built to cover a realistic number of sessions rather than being sized for a single visit, and how far it stretches depends heavily on your own re-chalking habits and how humid the room is.

What actually shortens how far a bag goes

Over-chalking is the biggest factor. Applying more than a thin, even coat doesn't add grip past a certain point, it mostly ends up on the floor or the hold, which means you're using chalk without getting anything back for it. Humidity also plays a role, since damp conditions mean more frequent reapplication regardless of how much you use each time.

What actually stretches a bag further

A blend that lasts longer per application means fewer chalk-ups overall, which naturally uses less chalk across a session even without changing how much you apply each time. This is part of why the ratio of chunky to fine matters beyond just performance, it affects how often you're reaching for the bag in the first place.

The takeaway

There's no fixed gram count that applies to everyone. A light, even coat, reapplied only when your hands actually need it, uses less chalk and typically performs better than chalking up on a fixed habit or piling it on out of caution.