Longitudinal reinforcement limits for columns per ACI 318-19

Two rules bound the vertical steel in a nonprestressed concrete column: the reinforcement ratio must sit between 1% and 8% of the gross area (§10.6.1.1), and there must be a minimum number of bars that depends on how they are confined (§10.7.3.1). This page states both, explains why they exist, and shows where real designs land.

The steel ratio: 1% to 8% (§10.6.1.1)

0.01\,A_g \;\le\; A_{st} \;\le\; 0.08\,A_g \qquad \rho_g = \frac{A_{st}}{A_g}

Why a minimum: under sustained load, creep and shrinkage move compression from the concrete into the steel. With too little steel the bars can yield under service load. The minimum also gives some bending resistance for moments the analysis did not capture.
Why a maximum: above 8% the bars crowd the section so badly that concrete cannot be placed and consolidated around them.

Minimum number of bars (§10.7.3.1)

Transverse reinforcement	Minimum bars
Rectangular or circular ties	4
Spirals	6

Four bars puts one bar in each corner of a rectangular tie; spiral columns need at least six. See spiral reinforcement requirements for the rest of the spiral rules.

Steel area limits for common sections

Section	A_g (in²)	Min A_st, 1% (in²)	4% (in²)	Max A_st, 8% (in²)
12×12 in	144	1.44	5.76	11.52
16×16 in	256	2.56	10.24	20.48
20×20 in	400	4.00	16.00	32.00
24×24 in	576	5.76	23.04	46.08
20 in diameter	314.2	3.14	12.57	25.13

Bar areas for reference: #6 0.44, #7 0.60, #8 0.79, #9 1.00, #10 1.27, #11 1.56 in².

Practical notes

8% is really 4% where bars are lap spliced. At a lap splice both bars run side by side, so a column at 4% becomes 8% through the splice zone if every bar is spliced at the same level. Most designers treat about 4% as the practical ceiling for that reason, and 1% to 2% is often the economical range because concrete is the cheaper way to carry compression.
Bar spacing can govern before 8% does. The clear distance between longitudinal bars must be at least the greatest of 1.5 in, 1.5d_b and 4/3 of the maximum aggregate size (§25.2.3). On small sections this, not the ratio, limits how much steel fits.
Oversized columns. Where a column is larger than strength requires, for example for architectural reasons, ACI permits basing the 1% minimum on a reduced effective area (§10.3.1.2), with the strength reduced to match. StructurePoint's spColumn, for example, scales the capacity down in that case and does not accept steel below 0.5% of the gross area.
Seismic (Chapter 18) detailing is outside calcnote's scope.

Where real designs land

Steel ratios computed by calcnote's engine for the sections on this site:

Section	A_st (in²)	ρ_g	Result
14×14 in, 4-#7 (Wight Example 12-2)	2.40	1.22%	Within limits
16×16 in, 8-#9 (live demo)	8.00	3.13%	Within limits
20 in diameter, 8-#10 (spiral)	10.16	3.23%	Within limits
16×16 in, 4-#6	1.76	0.69%	Rejected: below 1%

For the last row calcnote stops before any capacity calculation and says what would fix it: at least 2.56 in² of steel, or a section no larger than 176 in² (about 13×13 in) for the same bars.

What calcnote checks

Every run checks 0.01 ≤ ρ_g ≤ 0.08 on the gross section and the §25.2.3 clear spacing between bars, and only offers bar counts that meet §10.7.3.1 (4, 8, 12, 16 or 20 bars for rectangular sections; 6 or more for circular). Scope note: the reduced effective area option, lap splice congestion and seismic limits are the designer's call.

Sources

ACI 318-19: §10.6.1.1 (1% and 8% limits), §10.7.3.1 (minimum bar count), §10.3.1.2 (reduced effective area), §25.2.3 (clear spacing of column bars).
StructurePoint, spColumn Manual, v10.30: architectural column option and the 0.5% floor.