RSA Calculator Tool

enter two primes:	$p$ $q$	large random primes $p \neq q$
$\downarrow$		RSA modulus $n = p \times q$
$\downarrow$		totient $\phi(n) = (p-1)(q-1)$
$\downarrow$		$1 < e < \phi(n)$ $e$ coprime with $\phi(n)$
$\downarrow$		$1 < d < \phi$ $ed \equiv 1$ $mod$ $\phi$
enter message number:	$\Delta t = $	$c \equiv m^e \ (mod \ n)$
enter ciphertext number:	$\Delta t = $	$m \equiv c^d \ (mod \ n)$

Notes: $p$ and $q$ must not be equal. The message number must be strictly less than $(n-1)$.
Using large primes will result in large values for the exponent $d$. Values of $d > 10^6$ are going to take forever to decrypt, but if you wait long enough it should eventually complete.
Using large primes won't work on all browsers because of the larger-than-usual memory allocation requirement. Chrome works best; Chrome doesn't care how much memory you ask for.
Here are some primes you can try for testing.