Overview
Same function, different calculator layouts
The most common scientific calculator gives you a direct button for combinations and a direct button for permutations. Other models hide those functions behind a or menu, or print them as a secondary label that needs or . RPN calculators are different again: you usually place n on the stack, press , type r, and then run the combination or permutation function.
No matter the calculator style, the meaning of the inputs stays the same: n is the total number of available items, and r is how many you are choosing or arranging.
Button guide
What the buttons usually look like
Direct function keys
Combination buttons
Choose these when order does not matter. That means you only care which items were chosen, not the order they are listed in. If ABC and CBA represent the same group, you want a combination button such as .
Permutation buttons
Choose these when order does matter. That means switching positions creates a different result. If ABC and CBA should count as two different arrangements, you want a permutation button such as .
This is still the easiest calculator style overall: type n, press the correct function, type r, then evaluate.
Menu-based calculators
You will see this grouping when the calculator does not give combinations and permutations their own front-facing keys. Open or the probability area, choose , then pick or after entering n.
Shifted or second-function keys
This grouping appears when the calculator prints or above another button instead of giving it its own key. Enter n, press or , press the marked key, then enter r.
RPN stack entry
You will see this grouping on Reverse Polish Notation calculators. The common pattern is n, then , then r, then or . Many RPN models return the answer immediately without a final equals key.
Inputs
What these functions expect you to enter
- For combinations: type n first, then the combination function, then r. Example: .
- For permutations: type n first, then the permutation function, then r. Example: .
- Remember the meaning: n is the total pool, and r is the number selected or arranged.
Most common style
Direct nCr and nPr buttons
Combination example
This is the fastest layout. Type n, press the function, type r, then evaluate.
Permutation example
Use this when order matters, such as ranking winners or arranging items in distinct positions.
Menu style
Calculators that hide the function in a menu
Menu-based combination example
- Type for n.
- Open , then the or probability submenu.
- Select .
- Type for r.
- Press .
Shifted style
Calculators with a secondary nCr or nPr label
Shifted permutation example
- Type for n.
- Press or .
- Press the key with printed above or beside it.
- Type for r.
- Press .
RPN style
Reverse Polish Notation calculators use stack order
RPN combination example
On RPN, the common pattern is n, then , then r, then the function.
RPN permutation example
Many RPN models execute immediately after the function key, so there may be no final .
Common mistakes
What usually causes the wrong answer
- Entering r first. Most calculators want n first and r second.
- Using when order matters, or using when order does not matter.
- Forgetting to open the menu or use on calculators that hide the function.
- On RPN models, forgetting the middle before typing r.