How deep will your Mouthfill take you.

sfsdiving
Feb 5
2 min read

Understanding Mouthfill Factor in Freediving

When we talk about deep equalisation, one number matters more than most: Mouthfill Factor (MF). It’s a simple way to understand how hard your mouthfill has to work as you descend.

What Is Mouthfill Factor?

Mouthfill Factor is calculated by dividing the absolute pressure at your maximum depth by the pressure at the depth where you took your mouthfill.

MF = Pressure at max depth ÷ Pressure at charge depth

Because pressure increases by 1 bar every 10 metres:

Pressure (bar) = Depth ÷ 10 + 1

Example 1 — Surface Charge

A diver charges their mouthfill at the surface (0 m) and can make it last to 35 m.

Pressure at 0 m = 1 bar
Pressure at 35 m = 4.5 bar

MF = 4.5 ÷ 1 = 4.5

That means the air shrinks to just 22% of its original volume. This is a high MF, which is why surface mouthfills don’t last very deep.

Example 2 — Same MF, Deeper Charge

Now the diver keeps the same MF (4.5) but takes the mouthfill at 25 m.

Pressure at 25 m = 3.5 bar

Target pressure = 3.5 × 4.5 = 15.75 bar

Convert back to depth:

(15.75 − 1) × 10 = 147.5 m

So in theory, that same quality of mouthfill could now last to around 148 m — simply because it was taken deeper.

Example 3 — Improving Charge Depth

A diver can currently take a surface mouthfill and make it work to 20 m.

Pressure at 0 m = 1 bar
Pressure at 20 m = 3 bar

MF = 3 ÷ 1 = 3

Now that diver improves their technique and can take their mouthfill at 15 m instead, while keeping the same MF (3).

Pressure at 15 m = 2.5 bar

Target pressure = 2.5 × 3 = 7.5 bar

Convert back to depth:

(7.5 − 1) × 10 = 65 m

By moving the charge from the surface to 15 m, their potential depth jumps from 20 m to about 65 m — without changing lung volume, just improving timing and technique.

Why This Matters

A deeper charge

Air shrinks less during the dive
Equalisation feels easier
You have more usable volume at depth

A shallow charge means:

Air disappears fast
Technique must be extremely efficient
Small leaks or tension cost you the dive

This is why experienced divers aim to take their mouthfill as deep as safely possible — not to pack more air, but to reduce the pressure change that air has to survive.

In deep diving, efficiency isn’t just about how much air you take — it’s about when you take it.

Mouthfill learning plan (simple)

While learning: Only charge at the surface or ~5 m.
- Reason: It’s easier to control, and you can feel full cheeks/jaw/tongue without stress.
Once consistent and comfortable: Start trying charges at 10–15 m.
- Goal: Learn to manage compression and keep the glottis sealed.