Will your action figure fit on this shelf?

Measure the space inside the shelf and the figure in its display pose. Include anything that sticks out, from a foot behind the base to the tip of a raised hand. The calculator gives the space left on each axis.

Measure the narrowest part

Measure from the shelf surface to the lowest obstruction. For width, use the gap between neighbouring pieces. For depth, close the cabinet door first and measure to its inside face. A hinge or door frame can also narrow the opening as you put the figure in.

Try a measurement you already know

A figure 150 mm tall in a space 180 mm high leaves 30 mm above it. Width and depth still need their own measurements. Tamashii Nations lists the pictured Dragon Ball GT Super Saiyan 4 Son Goku at approximately 150 mm. A different pose or a riser can change the height you need. Tamashii Nations · Toy Snowman — Son Goku

Read the guide to figure scale and stated height