Effective depth (d) vs overall depth (h): why the distinction matters
The two numbers look similar and are easy to mix up, but a flexural design calculation genuinely needs the one measured to the reinforcement — not the one measured to the edge of the concrete.
Overall depth (h) is simply the full physical depth of a concrete section — the number on the formwork drawing, measured from the top face to the bottom face. Effective depth (d) is a different, smaller number: the distance from the extreme compression fibre (usually the top of the section, for a normal sagging beam) to the centroid of the tension reinforcement. The gap between h and d isn't a rounding difference — it's the concrete cover, plus any stirrup or tie bar in the way, plus half the diameter of the main bar itself, all of which physically sit between the very edge of the concrete and the actual centre of the steel.
That gap exists for real reasons, not by accident: concrete cover protects reinforcement from corrosion and gives it fire resistance, and a minimum cover is itself a code requirement (durability provisions in EC2, cover requirements in ACI 318) that varies with exposure conditions. Reinforcement genuinely cannot sit flush at the very edge of a concrete section and still do the job cover is there for.
Why flexural calculations use d, not h
A beam or slab resists bending as a couple — a compression force near the top of the section and a tension force at the reinforcement, separated by an internal lever arm that's built directly from d, not h. Using the full overall depth in place of effective depth overstates that lever arm, which overstates the section's real moment capacity — a mistake that tends to happen precisely because h is the more visible, more obviously-labelled dimension on a drawing, while d has to be worked out from the section's actual reinforcement layout.
This is also exactly why d isn't a fixed property of a section shape alone — it depends on the reinforcement layout too. A section with two layers of bars (rather than one) has its second layer sitting further from the compression face, which pulls the effective depth down for that layer; multi-layer reinforcement genuinely changes d, not just how much steel there is. RC Beam Design, RC Slab Design, RC Column Design and Foundation Design all compute effective depth from the actual reinforcement arrangement you set up, rather than assuming it from the overall section size alone (see the related tools below).