Understanding the Importance of Mash Tun Dead Space in Brewing Process

The short answer: dead space is the wort that stays in the mash tun (or kettle) below the drain outlet or false bottom, so it never reaches the next vessel. It lowers your volume and efficiency unless you account for it. Measure it once: fill the empty vessel with water, drain it as you would on brew day, and measure what’s left. Then add that volume to your water calculations. Bottom-drain vessels leave almost none.

Cause	Fix
Side-mounted outlet	Add a dip tube, or use a bottom-drain vessel
Space under a false bottom	Measure it and add it to your strike water
Hoses, pump and plate chiller	Measure what they hold and add it to your sparge or boil volume
Grain absorption	Separate from dead space: about 0.1 to 0.125 gal per lb of grain (0.8 to 1.0 L/kg)

Why dead space matters

• It costs volume. You collect less wort than you planned, and end up with less beer or a lower gravity if you top up with water.
• It costs sugar. The wort left behind has sugar in it. How much depends on how you sparge. With a fly sparge, the last runnings are weak, so little sugar is lost. With a batch sparge, you lose wort at second-runnings strength. With no sparge, you lose full-strength wort: if 0.5 gal (1.9 L) of 7 gal (26.5 L) of wort stays behind, you lose about 7% of your sugar.
• It throws off the mash. Water sitting under a false bottom isn’t in the grain, so the mash is thicker than you planned unless you add that volume to the strike water.

How to measure it

1. Set up as on brew day: vessel level, false bottom in, pump and hoses connected.
2. Add water in measured steps of 1 qt or 1 L until it just covers the false bottom. That’s the volume under the false bottom.
3. Add a few more gallons, then drain it exactly the way you do on brew day.
4. Measure what’s left in the vessel. That’s your dead space. Then drain the pump and hoses into a jug: that’s your line loss.
5. Repeat for the boil kettle. On brew day the kettle keeps more, because hops and trub hold wort too, so check it again on a couple of real batches.
6. Enter the numbers in your brewing software, as mash tun dead space and kettle loss.

How to reduce it

• Use a bottom-drain vessel. A floor that slopes to a centre drain leaves almost nothing behind. Our DrainPro mash tun drains from the very bottom.
• Fit a dip tube to a side outlet so it picks up from the lowest point of the vessel.
• Fly sparge, so whatever stays behind is weak wort.
• Whirlpool the kettle so hops and trub settle in a cone in the centre, and drain the clear wort from around it.
• Keep hoses short and drain the pump and chiller at the end of the transfer.

Dead space on single-vessel systems

On a single-vessel system like the Unibräu, the mash and the boil happen in the same kettle. The wort under the grain basket never has to be transferred, so mash tun dead space costs you nothing. It’s part of the volume that recirculates through the grain during the mash.

The loss comes after the boil: whatever stays in the kettle, pump, hoses and chiller when you transfer to the fermenter. The Unibräu DrainPro kettle has a sloped floor to a welded bottom drain, so trub collects in one spot and the kettle empties completely. The Brew Day Planner works out water volumes for each Bräu Supply system.

"Dead space" that isn't lost

Some all-in-one makers list several gallons of "dead space." Spike, for example, lists 7.7 gal for its 20 gal Solo with the kettle empty (Spike help centre, checked October 8, 2026). That figure is the space outside its grain basket (shown in yellow on Spike’s diagram): water that sits outside the grain during the mash. It never leaves the kettle, and all of it goes into the boil, so none of it is lost.

The trouble starts when that number goes into brewing software as dead space. The software treats it as wort you lose, and your water and efficiency numbers come out wrong. BeerSmith users ran into exactly that with the Solo profile (BeerSmith forum).

True dead space is wort that can't leave the vessel. A kettle that drains from the lowest point of its floor, like the Unibräu DrainPro, has next to none. What you lose is the wort left in the hoses, pump and chiller, plus the trub you choose to leave behind.

What to enter for a Unibräu

• Mash tun dead space: 0. The wort under the basket stays in the kettle and goes into the boil.
• Kettle loss (Brewfather calls it trub/chiller loss): what stays in the kettle, pump, hoses and chiller after you transfer to the fermenter. Measure it once with water, then fine-tune it after a couple of real batches.
• Grain absorption: about 0.1 to 0.125 gal per lb of grain (0.8 to 1.0 L/kg).

Dead space and the other losses

Loss	Where the wort goes	Add it to
Mash tun dead space	Stays below the outlet or false bottom	Strike or sparge water
Grain absorption	Soaked up by the spent grain	Total water
Line loss	Pump, hoses and chiller	Sparge or boil volume
Kettle loss	Left with the hops and trub	Boil volume
Boil-off	Steam	Pre-boil volume

Frequently asked questions

Should I add dead space to my strike water?

If your mash tun has a false bottom, yes: add the volume under the false bottom, so the grain gets the amount of water you planned.

Is dead space the same as trub loss?

No. Dead space is wort left in the mash tun below the outlet. Trub loss, or kettle loss, is wort left in the boil kettle with the hops and break material. Both cut what reaches the fermenter.

Can I just top up the fermenter with water?

You can, but it lowers the gravity. It’s better to plan the extra water before you mash.

Does dead space lower efficiency?

Yes. Wort left in the mash tun lowers mash efficiency, and wort left in the kettle and lines lowers brewhouse efficiency. See mash efficiency.

Does a bottom-drain kettle have dead space?

Next to none in the kettle itself, because it drains from the lowest point. You still lose what sits in the hoses, pump and chiller, and the trub you leave behind on purpose.

Related guides

Lautering and sparging, no-sparge brewing, HERMS brewing explained and scaling recipes.