Any calculator that estimates what a payables process costs, or what automating it would save, is arithmetic wrapped around five inputs. If the inputs are industry averages, the output is an industry average with your logo on it. If they are measured in your own team, the output is a figure you can defend in a meeting, which is the entire point of doing the calculation.
The three cost inputs
Monthly invoice volume, median minutes from arrival to approval timed rather than estimated, and the fully loaded hourly cost of the people doing that work. Fully loaded means salary plus employer costs divided by worked hours, not the figure on a payslip. These three give your current spend on the mechanical part of payables.
The input that caps the saving
The share of invoices arriving with both a purchase order and a recorded goods receipt. An invoice with nothing to match against needs a person, so the saving applies only to the matchable share. Calculators that omit this input overstate the result, and the overstatement is exactly what a sceptical reader will find.
The cost side
What the software would cost per month, including implementation amortised over a sensible period. Subtracting it gives a net figure. A calculator that reports gross saving without the cost is not answering the question anybody is actually asking, which is whether this is worth doing.
Questions people ask about accounts payable calculator
How do we measure minutes per invoice?
Time ten invoices end to end including interruptions, and take the median rather than the fastest.
How do we get purchase-order coverage?
Sample a hundred invoices from last month and count how many had both an order and a receipt behind them.
Should we include exception handling time?
Measure it separately, because it is the part automation touches least and blending it hides that.