Homemade Mechanical Calculator that Works, Knock on Wood
Your Calculator Doesn't Know Math?
Will Gerhardinger spent about eight years of his free time building a fully functional calculator out of plywood. He cut every gear, sprocket, and cam on a scroll saw. The wood is mostly Baltic birch. The metal shafts came from Home Depot. There is a hand crank and no battery... it adds, subtracts, multiplies, and divides.
It also explains something most people get wrong about calculators, including the one on your phone.
A 350-Year Head Start
Blaise Pascal built a geared adding machine in the 1640s.
Gottfried Leibniz designed a more ambitious one in the 1670s, with stepped drums that made multiplication practical. Leibniz paid a Paris clockmaker named Ollivier for three weeks of work. The job took the two of them about forty years, and the machine never lived up to the hype.
Charles Xavier Thomas finally made the idea sell in the 1820s. His Arithmometer became the first commercially successful mechanical calculator.
...Will’s wooden machine is based on the Arithmometer.
How a Plywood Machine Does Arithmetic
You set a number on the sliders. You turn the crank. The machine adds that number into the accumulator at the top.
Multiplication is repeated cranking. To compute 412 × 55, you turn the crank five times, slide the accumulator over one decimal place, and turn it five more times. That’s ten turns, not 55.
Subtraction works by flipping a lever. It shifts a set of bevel gears so the number wheels spin the other way.
The hard part is the carry. Add 1 to 999,999 and every column has to roll over, one after another. That’s trivial for you and a real problem for gears.
Gerhardinger’s fix is a chain linking nine sprockets, each offset from its neighbor by one tooth. The carries ripple through in order instead of all at once.
Your Phone Does the Same Thing, Faster📱
A digital calculator doesn’t store a table of answers. It converts numbers to binary and runs them through logic gates, which only understand on and off.
Carrying is still the hard part. The standard circuit for adding binary numbers is literally called a ripple carry adder. Chips also multiply by shifting and adding, the same trick as sliding that accumulator over a decimal place.
Rules matter too. Press 2 + 3 × 4 on two different calculators and you can get 14 or 20. One follows the order of operations and one doesn’t. Same buttons, different rules, different answer.
A calculator never knows the answer. It follows the rules built into it.
With a Mechanical Calculator, the Rules Are the Product
A phone calculator has rules for arithmetic. It has none for depreciation.
Since the One Big Beautiful Bill Act, 100% bonus depreciation is permanent. A cost segregation study decides how much of a building’s basis moves into bonus.
Our bonus depreciation calculator builds those rules in. Pick a scenario from Conservative (around 15%) up to High Reclassification (35 to 40%), enter your property price, and see your first-year deduction in seconds.
Take a look at these vintage calculator ads below; it’s amazing to see how far we’ve come!
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