Minimum Beam Depth Calculator — ACI 318-19 Table 9.3.1.1
ACI 318-19 lets you skip explicit deflection calculations if your beam meets the minimum depth requirements of Table 9.3.1.1. This page explains the four span conditions, gives you the formulas, and links to a free calculator for instant sizing.
Formulas
| Quantity | Formula |
|---|---|
| Simply supported | h_min = ℓ / 16 |
| One end continuous | h_min = ℓ / 18.5 |
| Both ends continuous | h_min = ℓ / 21 |
| Cantilever | h_min = ℓ / 8 |
| Modification for f_y ≠ 420 MPa | Multiply by (0.4 + f_y / 700) |
| Modification for lightweight concrete (1440 ≤ w_c ≤ 1840) | Multiply by max(1.65 − 0.0003·w_c, 1.09) |
How to use it
- Identify the support condition. Look at the structural framing: is the beam end pinned, monolithically cast with a column, or sticking out?
- Pick the right ratio from Table 9.3.1.1 — ℓ/16, ℓ/18.5, ℓ/21, or ℓ/8.
- Enter span ℓ (the clear span for cantilevers; centre-to-centre for simply supported).
- Apply the f_y modifier if f_y ≠ 420 MPa (rebar grade other than Grade 60).
- Apply the lightweight modifier if applicable.
- If your final h is less than the table value, you must perform a full deflection check per ACI §24.2.
Frequently Asked Questions
Is this required even for shallow beams that I know will be fine?
No — Table 9.3.1.1 is a deemed-to-comply shortcut. If you're willing to do a deflection calculation per §24.2, you can use any depth. The table just lets you skip the analysis.
What if I'm using Grade 80 or Grade 100 rebar?
Multiply the table value by (0.4 + f_y/700). For f_y = 550 MPa: factor = 1.186. For f_y = 690 MPa: factor = 1.386.
Does this apply to slabs?
No — slabs use ACI Table 7.3.1.1 (one-way) or §8.3.1 (two-way). Different ratios apply.
What about T-beams?
Same Table 9.3.1.1 applies — but the depth is measured to the bottom of the stem, including the flange.
Do these limits also satisfy stiffness for vibration?
Not necessarily. Vibration governs in long-span composite floors and cantilever balconies; check L/D ratios against AISC Design Guide 11 if vibration is a concern.