It should be at least 12 inches thick.
I plan on making a small deck and I live in Howard County Maryland. The deck code here says that the concrete footing must be 30 in below ground level and have a thickness of at least 8 inches (i.e. to 22 inches below ground level). This which means the wood post is in the ground.  A typical club-foot slab has 12 thick edges and the 12 thickness usually is installed with a 'rebar beam' structure around the perimeter, and is about 24-30 wide and then bevels in to the 6 thickness. The extra concrete carries the weight of the walls and structure.  B. Minimum Foundation Wall and Wall Footing Thickness. For masonry or concrete construction, the minimum foundation wall will be 6 inches. The minimum reinforced concrete footing thickness will be 6 inches or 1-1/2 times the length of the footing projection from the foundation wall, whichever is greater. 503-2. PIER AND COLUMN FOOTING REQUIREMENTS. Footing Requirements. At the same time as your slab is poured, you will also need to pour perimeter footings, as illustrated below. This is called a monolithic pour or monolithic slab. Perimeter footings should be 12″ wide, and 12” deep. For example, if you have purchased a 20′ wide by 40′ long building, your slab should be 20′ wide by 40′ 2″ long. Your floor should be a minimum of 4” thick. The concrete’s strength should be a minimum of 2500 psi, with fiber mesh reinforcement added at the batch plant OR #3 rebar on 24” centers. The thickness of the foundation walls you are looking at are not the same. The wall below and the one to the left of the hose bib faucet are only 8 inches thick. The other walls are 10 inches thick even though there is less soil pressing against them. There are some basic rules of thumb for foundation wall thickness and these are outlined in many modern building codes. Poured concrete foundation walls that are less than 8 feet tall and have soil outside that is 6 or 7 feet deep against the wall can often be 8 inches thick and function quite well. As soon as you go higher or have greater depths of soil pushing against the wall, you need to increase the thickness to 10 inches Typically, such (continuous) footings should be a minimum of 8 thick to a 12 deep by 24 wide base that extends at least 12 below the frost line, resting on undisturbed sub-grade and assuming good soils for bearing. Poor bearing soils will require a wider base. Reinforcement is required and so forth.  3. Height (vertical thickness) of the concrete footing. I pour my concrete footings thick when building mortared stone walls; that is, I make them at least 12 inches in height. If I’m building a really big wall, I’ll pour them thicker (taller) because the concrete footing will need to support more weight. So, for example, a 32-inch-wide footing under an 8-inch wall would need to be at least 12 inches thick. Instead, however, you could reinforce the footing with transverse steel (running in the crosswise direction, not along the footing). In most residential situations, #4 rod at 12 inches o.c. will be plenty for 8-inch-thick footings up to 4 feet wide. The steel should be placed about 3 inches up from the bottom of the footing. You can leave the concrete footing a few inches below grade to make sure it doesn’t show when the wall is finished. 4. Width (horizontal thickness) . Your concrete footing should be a little wider than your mortared stone wall. If your wall will be 18 inches thick, you should make your concrete footing 24 inches thick. That way you know all of the wall will be supported, and you’ll have a little wiggle room in case your layout was a little off. 5. I pour my concrete footings thick when building mortared stone walls; that is, I make them at least 12 inches in height. If I’m building a really big wall, I’ll pour them thicker (taller) because the concrete footing will need to support more weight.
http://diy.stackexchange.com/questions/27653/how-should-i-build-the-concrete-footings-for-my-deck https://answers.yahoo.com/question/index?qid=20070303095938AAB6dSa http://portal.hud.gov/hudportal/documents/huddoc?id=49303gc5GUID.pdf https://www.absoluterv.com/concrete-foundation/ http://www.askthebuilder.com/thickness-of-poured-concrete-foundation-walls/ https://answers.yahoo.com/question/index?qid=20090729041119AA8cEE8 http://landscaping.about.com/od/Building-Stone-Walls/a/concrete-footings.htm https://pro.homeadvisor.com/article.show.Footing-Fundamentals.13700.html http://landscaping.about.com/od/Building-Stone-Walls/a/concrete-footings.htm
how thick does a concrete footing need to be