For US imperial reinforcing bars, the three standard hook angles (90, 135, and 180 degree) are not interchangeable: the finished bend diameter D, the straight extension A or G, and the resulting J length all scale with bar size from #3 through #18, as tabulated by NCDOT and the Masonry Society for all grades of steel [S1][S3].
The dimension that controls the whole geometry is the inside bend diameter, called D in the standard bar hook tables and the "finished bend diameter" in industry shorthand; D grows from 2 1/4 in. at #3 to 2 ft-00 in. at #18, and the 90, 135, and 180 degree A or G extensions follow that scaling [S1][S2].
What D, A/G, and J actually mean
Three letters govern every standard rebar hook: D is the finished inside bend diameter in inches, A or G is the straight leg extension beyond the bend measured to the outside of the bar, and J is the total developed length of the hook used for lap and anchorage calculations, with 180 degree J tabulated separately for #10, #11, #14, and #18 main bars [S1][S2]. For a #5 bar the geometry reads 90 degree: D = 0-03, A or G = 0-07, while 180 degree reads D = 0-03, A or G = 0-05 3/4; the 90 degree leg is the longer one because it has to develop the same tension force without the closing bend of the 180 [S2].
For a #8 bar the spread is wider: 180 degree uses D = 0-06, A or G = 0-11, 90 degree uses D = 0-06, A or G = 1-04, and the 180 J is not separately tabulated on the standard hooks table because the 90 and 180 columns are the ones listed for main bars in this size range [S1][S2]. The 180 degree hook is the most anchorage-efficient per inch of bar, which is why it dominates column ties, pile cage ties, and seismic boundary element confinement where anchorage space is tight.
Main-bar hook table, #3 to #18
The NCDOT 425 main-bar table covers all three angles across 11 bar sizes, with D in inches, A or G in ft-in, and J (where listed) for #10, #11, #14, and #18: a #3 reads 180 D = 2 1/4 in., A or G = 0-05; 90 D = 2 1/4 in., A or G = 0-03; 135 D = 2 1/4 in., A or G = 0-06; a #10 reads 180 D = 10 3/4 in., A or G = 1-05; 90 D = 10 3/4 in., A or G = 1-01 1/4; 135 D = 10 3/4 in., A or G = 1-10, with 180 J = 1-10 [S1][S2]. The trend is monotonic: a #18 closes the table with 180 D = 2-00, A or G = 3-00; 90 D = 2-00, A or G = 2-04 1/2; 135 D = 2-00, A or G = 3-05 [S2].
Two facts jump out of the main-bar table. First, D is identical for both the 90 and 180 angles on the same bar size, so the bending machine pin (not the angle) controls setup. Second, the 90 degree leg is consistently the longer A or G while the 180 degree is shorter, which is why fabricators and detailers prefer 180 degree hooks when form clearance along the straight run is the binding constraint.
Stirrup and tie hook table, #3 to #8
Stirrups and ties use a separate, smaller table because the bend diameter is allowed to drop to 4db for #3 through #8 bars, per the Masonry Society reinforced-masonry geometry, giving D = 0-02 for #3, D = 0-02 1/2 for #4, D = 0-03 1/4 for #5, D = 0-04 1/2 for #6, D = 0-05 1/4 for #7, and D = 0-06 for #8 [S1][S3]. The standard stirrup layout lists 90 degree: A or G = 0-04 (for #3) up to 1-04 (for #8), and 135 degree: A or G = 0-04 (for #3) up to 0-10 1/2 (for #8) [S1][S2].
The 135 degree seismic stirrup table tightens further because seismic confinement requires the hook to wrap back into the core: for a #5 bar, D = 0-02 1/2, 135 degree A or G = 0-05 1/2, with H (approximate, the leg bearing against the longitudinal bar) = 0-03 3/4, and for a #8 the same D = 0-06, A or G = 0-10 1/2, H = 0-06 [S1][S2]. That 135 degree seismic table is the one that drives ACI 318-19 special moment frame and shear wall boundary element detailing, not the 90 degree table.
Selection criteria, by application
Choosing between 90, 135, and 180 degree is a function of three numbers: required development length, available cover, and whether the hook is bearing against a transverse bar. 180 degree hooks win where the straight leg must stay short and the closing bend can supply the rest of the embedment; 90 degree hooks are the cheapest to bend because there is no return leg and the leg can run along formwork with no interference; 135 degree hooks are the only choice for seismic stirrups, beam-column joints, and any case where the hook must hook back over a longitudinal bar to prevent splice slip [S1][S2][S3].
For cost-driven precast and tilt-up work the fabricator will default to 90 degree on #4 and #5 because the small stirrup-class D lets the bending shop run small pins; for cast-in-place columns with #9 and #10 verticals, 180 degree is almost universal because J = 1-10 for #10 (per the main-bar table) is shorter than the 90 degree A or G of 1-01 1/4, and a tighter lap is the only way to keep splice lengths manageable [S1][S2].
Comparison at a glance: cost, anchorage, and integration
Compared across three decision criteria, the geometry lines up predictably. Cost per bend: 90 degree is the lowest (one bend, no return), 180 degree is the highest (two bends plus the closing arc), 135 degree sits in the middle and is the most fabricator-friendly for shop bending. Anchorage efficiency per inch: 180 degree is the highest, 135 degree is mid-pack, 90 degree is the lowest because the straight leg has to develop the full bar force. Field integration: 90 degree needs the most cover along the leg, 180 degree needs the least straight run but the most form depth to absorb the J, and 135 degree needs a transverse bar to hook back onto, which is why it is universal in seismic stirrups and rarely used elsewhere [S1][S2][S3].
For practical rebar specification, the 4db stirrup D and 6db main-bar D are the most-cited values in mill cert submittals, and they line up with the tabulated D entries above within the fabrication rounding used on most DOT projects [S1][S3]. If your hook detail does not match the table, the inspector will reject the piece.
Limits, failure modes, and standards to check
Two failure modes dominate real-world rebar hook problems. First, the inside radius of a small stirrup bend can crack when the bending shop over-pins, because the bar is on the lower edge of the 4db minimum and a worn pin can let the bar slip and neck; the fix is to upsize D to 5db or to drop one bar size. Second, the 135 degree seismic stirrup leg H is "approximate" in the table, meaning fabricators routinely cut the closing leg short, and the inspector will reject pieces where H drops too far below the listed value because the splice can pull out under cyclic load [S1][S2].
The governing standards are ACI 318-19 (Chapter 25 for hook geometry, 4db stirrups, 6db and 8db main-bar D), ACI 117 for tolerance, and the project-specific DOT or IBC tables that override the ACI defaults. The standard hooks geometry for stirrups mirrors the ACI 4db rule for #3 through #8 and is the right reference for CMU cells, not cast-in-place concrete [S1][S3]. For a deeper look at the rebar categories these hooks are used in, see the encyclopedia entry on rebar and the related article on where ACI 318-19 permits Grade 100 deformed rebar, which maps the same D and A or G minimums against the higher yield grade.
Next spec cycle, watch for ACI 318-25 to keep the 4db stirrup rule and to revisit the 180 J for #14 and #18; on the material side, mill cert submittals are now routinely listing D as a calculated value rather than a tabulated one, which will shift some of the inspection burden from the bending shop to the QA reviewer.