According to SP: 27-1984, thickness of slab or RC slab shall be measured to nearest of:

Explanation:

General rules for measurement are:

All works shall be measured subject to the following tolerances:

(i) Dimensions shall be measured to the nearest 0.01 meter.

(ii) Areas shall be measured to the nearest 0.01 sq.m.

(iii) Cubic contents shall be worked up to the nearest 0.01 cum.

Except: 

1. The woodworks should be measured nearest to 0.002 m.

2. Thickness of slab which shall be measured to the nearest 0.005 m.

3. Structural steelworks to the nearest 0.001 m. 

4. Thickness of roadwork (where the thickness is less than 200 mm) to the nearest 0.005 m.