You can use Polydojo forms to perform a large variety of on-the-fly calculations.

Adding Two Numbers:

For starters, let's simply add two numbers.

1. Add a 'Number' field labeled "Enter X" and check 'Show more configuration options.'.
   
2. Against 'Variable', type X (in caps) and click 'Done'.
   
3. Similarly add another 'Number' field labeled "Enter Y" and set 'Variable' to Y (in caps).
4. Add a 'Calculation' field labeled "The sum of X and Y is".
5. Set 'Expression' to X + Y (in caps); and finally click 'Done'.
   

That is it! Your form will now automatically add two numbers! Let's check it out.

Try It Out!

1. Click Save > Back to forms > Select the form you just saved > Fill.
2. The form will open in a new tab. Start typing some numbers!
3. You'll see that as you type, "The sum of X and Y is" is auto-calculated.

Going Beyond Simple Addition:

Now instead of X + Y, you could've set Expression to:

1. X - Y (Subtraction)
   
2. X / Y (Division)
   
3. X * Y (Multiplication)
4. X*X + Y*Y (Sum Of Squares)
   

You could add another 'Number' field labeled 'Enter Z', with variable Z (in caps). Then, you could use compound expressions such as:

1. X * (Y + Z) (Add, then multiply)
   
2. (X + Y + Z) / 3 (Average)
3. X * (1 - 10/100) or  X * 0.90 (10% Off)
4. Y * (1 + 18/100) or  X * 1.18 (18% extra, for tax etc.)