One diameter does not fit anything
What people want is a single diameter, and there is not one in either state's code. Virginia's Table R507.3.1 runs from a 7 inch square or 8 inch round footing at the small end to a 35 inch square, 40 inch round, 15 inch thick footing at the large end. That is a five to one spread in width inside one published table. Any answer that comes back as one figure is wrong no matter which figure it picks.
The 24 by 24 by 12 that circulates online is not a row in either table. Twenty four inches turns up in Virginia's table as a square side at a handful of load, area and soil combinations, and in North Carolina 24 by 24 is the largest row there is, good for 150 square feet of deck per post. Neither is a minimum and neither is a default.
Depth is a separate question with a separate answer and it is not this page. The short of it is that Virginia routes deck footings out to their own section, R507.3, whose depth rule starts at 12 inches below the undisturbed ground surface and pulls in frost protection where a deck is attached to a frost protected structure. How that frost figure gets set, and who sets it, is the entire subject of our page on footing depth in Virginia.
The three inputs the Virginia table wants before it will answer
R507.3.1 Minimum size states it in one sentence: the minimum size of concrete footings shall be in accordance with Table R507.3.1, based on the tributary area and allowable soil bearing pressure in accordance with Table R401.4.1. The table needs a load, an area and a soil class, and it will not answer with less.
Load first. R507.1 says a deck is designed for the live load required in Section R301.5 or the ground snow load indicated in Table R301.2, whichever is greater. The residential deck live load is 40 psf. The footing table offers 40, 50, 60 and 70 psf row groups, which means that in a jurisdiction whose published ground snow load runs above 40, the snow figure takes over and the lookup moves to a heavier row group.
Tributary area second, and this is the input almost nobody arrives having thought about. The table's own footnote defines it as the area, in square feet, of deck surface supported by post and footings. It is not the size of the deck. It is the slice of deck one individual post carries, so the post layout somebody drew is what decides your hole. Fewer posts under the same deck means more area on each one, which means bigger holes and more concrete.
Soil third. Three columns, 1,500, 2,000 and 3,000 psf or more, taken from the presumptive bearing values in Table R401.4.1. The clay and silt classes sit in the 1,500 column. Sand, silty sand, clayey sand, silty gravel and clayey gravel are 2,000. Sandy gravel and gravel are 3,000. Rock is off the top of the chart. Which column a given yard belongs in is a site question, and the red clay we work in is a common and legitimate answer rather than a worst case.
Two posts under the same deck, two different holes
Take a 40 psf deck on clay, which is the 1,500 psf column. A post carrying 40 square feet of that deck needs a 14 inch square or 16 inch round footing, 6 inches thick. Move to a post carrying 100 square feet of the same deck and the table gives 22 inch square or 25 inch round, 8 inches thick. Same deck, same soil, same code edition, and the footing has gone from 14 inches square to 22, more than twice the bearing area.
That is why we ask for the post layout rather than the deck dimensions when somebody calls about footings. Four posts and eight posts under one deck are two different concrete orders and two different days of digging, and the smaller-sounding deck is not automatically the smaller pour.
Four footnotes on that table change what happens in the yard. You may interpolate between rows and you may not extrapolate past the ends. The footing has to be big enough that the post bears on it completely. A brick or block pier wants a minimum 2 inch projection of footing on all sides. And the thickness column applies only to plain concrete footings, so the moment reinforcing steel goes into one, thickness stops being a lookup and becomes a design question for whoever drew the plan.
North Carolina is not a variation on Virginia, it is a different table
North Carolina deleted Section R507 out of its residential code. Where the deck section would be, the code reads Deleted, see Appendix M. There is no R507.3.1 and no Table R507.3.1 in North Carolina at all, so every deck article citing those numbers for a job in Surry, Alleghany, Stokes or Wilkes is citing a provision that does not exist in that state.
What North Carolina has instead is Appendix M, Wood Decks, which says of itself that its provisions are adopted as part of the code. It is not a handout. The volume it sits in carries a North Carolina State Building Code Council date of December 12, 2017 in its front matter, it took effect January 1, 2019 with the rest of the 2018 code, and the Office of the State Fire Marshal still lists those 2018 volumes as the ones currently in effect.
AM102.1 and Table AM102.1 size a footing on tributary area alone. There is no soil bearing column anywhere in it. Five rows: 8 by 16 inches up to 36 square feet, 12 by 12 up to 40, 16 by 16 up to 70, 16 by 24 up to 100, and 24 by 24 up to 150. Cast in place thickness is 6 inches for the two smallest and 8 inches for the rest. Its footnotes put the support post in the center third of the footing and require the top of the footing to be level for full bearing.
Now set the two tables side by side, because this is what catches people working both sides of the line. A post carrying 40 square feet of a 40 psf deck is a 14 inch square footing in Virginia on 1,500 psf soil and a 12 by 12 in North Carolina. Same deck, two states, two published answers, arrived at by two different methods. Do not average them and do not carry one figure across the line.
What the hole bears on, and the 200 square foot figure everybody quotes
The code's general footing rule says footings shall be supported on undisturbed natural soils or engineered fill. Topsoil is not one of the material classes in the bearing table at all, so it carries no assumed value. And where top or subsoils are compressible or shifting, the code, in place of a complete geotechnical evaluation, has them removed to a depth and width that keeps the moisture content stable in each active zone, or stabilized there by chemical, dewatering or presaturation methods. A pier resting on the loose stuff at the top of a hole is the settled corner a homeowner notices later.
There is also a note under that bearing table putting a soils investigation in play the moment the building official determines in-place soils are likely to bear under 1,500 psf. That is a call made at the county desk. It is not one the deck builder makes and it is not one we make.
The 200 square foot figure people quote is two different rules wearing the same number, and neither says what they think it says. In Virginia, R507.3 Exception 2 excuses a free-standing deck from having footings at all, and it reaches only a free-standing deck where the joists bear directly on precast concrete pier blocks at grade without support by beams or posts, the deck area does not exceed 200 square feet, and the walking surface is not more than 20 inches above grade at any point within 36 inches measured horizontally from the edge. All three conditions travel together. Drop the 36 inch qualifier and you have quietly changed what the exception covers.
The other 200 square feet belongs to a permit exemption in the model residential code, R105.2, and that provision is in force in neither Virginia nor North Carolina. Virginia's own permit exemption list names no deck at all, which leaves only the building official's minor and ordinary discretion, and whether that official will accept the no-footing exception on your lot is theirs to decide as well. We are not going to tell you that you can skip footings on a deck. That answer is not ours to give.
Our part is the concrete. We form, set the steel and pour the footings at the size the approved plan and the official call for, and our trailer line pump runs 2.5 and 3 inch hose so material can reach holes in a back yard a truck will never get to. How the hose gets walked down a footing run is its own answer, and piers under a shipping container is the same arithmetic with a different load sitting on it.