I need to build a concrete base for a mail parcel box on the ground. The parcel box is 36.6cm deep, 49.6cm wide and 112.3 cm high. It weighs about 15kg.
EDIT, added this illustration:
Size will be an opinion mainly, but at least 5 to 8 cm(WxL) bigger than the box and about 8cm deep/thick.
Anchor bolts should be easy to use to anchor the box and will make the box easy to remove for maintenance.
The depth depends on your regional frost line. For europe we have 80-150cm (varies wildly per source, depends on your location) frost line. That means, to avoid frost-heave, the textbook says the foundation must be deeper than the frost line.
An foundation of say, 40x50x80cm, is 0.16m³. According to the TDS of some random brand of concrete, an 25kg bag of dry concrete yields about 12.5L cured, so you need 14 bags of dry concrete, or about 330kg.
My personal handwaving says that half of that depth for something like an letterbox is still in the within-safe-limits area, especially if you add about 10-20cm of gravel underneath to remove pooling water. You can also add fieldstones you found during the excavation.
To attach a letterbox to an concrete foundation I would use anchor bolts, sealed with caulk. Or, even better, chemical anchors to prevent water ingres.
To use significantly less concrete while still being deep enough to be stable: Rather than pouring a single massive block, dig four holes (one for each corner/mounting hole location) and pour 4 small, but deep, columns with an anchor bolt in each.
Is the box at risk of being struck by vehicles?
You might choose to have slightly more frangible anchors so they break first, minimising damage to your vehicle. Some hardwood dowells vertically with cross-pins inside the base should break off if your car backs into the mailbox.
On the other hand you might elect to use the strongest masonry anchors available, reinforce the whole base with a half-inch slab of steel, and even build an internal frame of rebar and tie that into your concrete base. The intention here is to make the vehicle into the crumple zone, not your mailbox.
Some parts of the world have regulations about positioning relative to the road, so check them out for your location.
Don't forget to extend the footing (either concrete or gravel or similar) in front so you have room to use the box without standing in mud. Because directly in front of the box will be an unavoidable high traffic area so you won't have ground cover (grass) there for long. If there's a door in back of the box that will also regularly be used then extend the footing there as well.
An alternative suggestion. To get depth (so more stable ground) and to keep the base of the metal off the ground (else it will rust in minutes, especially from condensation in the inside) could you attach it to one or two fence posts. Drill a drain hole in the bottom.
