Decimate

Remove a share of the triangles from the selected parts while keeping their shape. Choose a percentage or a target, and preview first.

Decimate thins out the triangles of a part while keeping its overall shape. It is the tool for a part that matters and is simply too dense.

It is lossy: the removed detail is gone from the mesh. It can be undone.

When to use it

On the heavy parts you found in How to read the numbers, after you have deleted what is not needed. For a whole scene at once, Fit to budget is easier.

Steps

Select the parts to reduce.
Open the card Reduce triangles in the right sidebar, under Reduce.
In the list beside Decimate, choose how much to remove: −25%, −50%, −75% or −90%.
Press Preview… under it. A table shows each part with its triangle count now and after.
Press Decimate. The button shows how many parts are selected.
Look at the parts. If they have lost too much, press CtrlZ and try a smaller percentage.

A triangle target instead of a percentage

Choose Target… in the list.
Type the number of triangles that everything selected may have together.
A line under the field says what will happen, in the form “12,400 → about 6,200 triangles”.
Press Decimate. Each selected part gives up the same share.

What it keeps

Normals, so the shading stays smooth.
UVs, the coordinates that place a texture on a surface.
Vertex colours.
Material ranges, for a part that uses more than one material.

Good to know

It runs in the background, several parts at a time. The window stays usable, and a long run can be stopped.
The reduced mesh is what gets saved and exported.
Decimate has no keyboard shortcut on purpose, because it is lossy. Find it in the card, in the … list or with the search.