The buried footing and the stem, with soil type changing the geometry, not the opinion.
Foundation, roof and structure. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Retaining Wall Calculator tool.
A retaining wall holds back soil, and the forces on it come from the ground rather than from the wall. That is why this calculator asks for the soil type before it will size a footing, and why it shows the width it derived and lets you overrule it. The material quantities are the easy part. The assumption behind them is the part worth reading. The tool measures a wall as a wall: footing concrete, the stem in blocks or concrete, weep holes and the drainage behind it. Footing width follows a rule tied to the wall height and the soil, which is a starting point rather than a design. Where the soil is unknown, the honest answer is to say so, and the page does that instead of picking a value that would look confident and be wrong.
A wall twelve metres long and one point eight metres high, retaining ordinary clay, derives a footing of about nine hundred millimetres wide by two hundred and fifty deep on the rule the page shows. That footing run is twelve metres by zero point nine by zero point two five, which is two point seven cubic metres of concrete, three point two with waste. The stem at one point eight metres by twelve is twenty one and a half square metres of face, which in blocks is about two hundred and ten units plus waste. Weep holes at two metre centres along the retained face give six, each with its pipe and gravel collar. Change the soil to loose sand and the derived footing widens, and the concrete rises with it, before you have changed a single dimension yourself.
The page derives it from the wall height and the soil type, and shows both the width and the rule it came from. As a rule of thumb a footing runs between half and two thirds of the retained height, wider for weaker soils, and that is where the derived figure lands. It is a starting point for a material order, not a design. Where an engineer has specified a width, enter it and the quantities follow your figure, not the rule.
At roughly two metre centres along the retained face, with a fall to the outside, a gravel collar behind each one and a filter fabric to stop the wall silting up. The calculator counts them from the wall length and adds the pipe and the collar. Weep holes are the cheapest part of the wall and the most expensive to leave out, because a wall that cannot drain holds water and then fails, so the count is worth checking against the drawing.
No. A retaining wall is a structure that fails suddenly when it fails, and the design depends on the soil, the water, the surcharge above it and the wall's own mass. The calculator measures the materials for a wall whose dimensions have been decided, and it flags the footing rule so the decision can be checked. Use it to order concrete and blocks, and use an engineer to decide how deep, how wide and how reinforced.
It changes the footing width the page derives, and therefore the concrete and the excavation. Clay holds a steeper face than sand, and loose granular soil needs a wider base for the same height. The switch is deliberately visible rather than buried, because a wall specified for one soil and built in another is a wall whose footing is the wrong size for the ground it is actually in.
It stops and asks. An unknown soil type is reported as a missing input, because a default would produce a footing width with nothing behind it and a material order that looks finished. The useful next step is a trial pit or a soil report, and until that exists the honest output is a list of what is still unknown rather than a number that would be repeated in a quotation.