I was bored, so I beefed this up a little.

on *:TEXT:*:#: {
  if ($1 == +convert) {
    if (!%no.flood. [ $+ [ $nick ] ] ) {
      if (!$2) { msg $chan ERROR $+ : Insufficient Parameters. SYNTAX $+ : +convert c/f <temperature> | halt }
      if ($2 isnum) { msg $chan ERROR $+ : Insufficient Parameters. SYNTAX $+ : +convert c/f <temperature> | halt }
      if ($2 isletter) { msg $chan ERROR $+ : Insufficient Parameters. SYNTAX $+ : +convert c/f <temperature> | halt }
      if ($2 = f) {
        if ($3 = 0) { msg $chan Converting 0 $+ $chr(176) Fahrenheit to Celsius: -17.77 $+ $chr(176) Celsius | set -u5 %no.flood. [ $+ [ $nick ] ] 1 | halt }
        if (!$3) { msg $chan $nick $+ , I need a temperature to convert. }
        if ($3 isletter) { msg $chan Converting $3 $+ : You\'re an Idiot? | set -u5 %no.flood. [ $+ [ $nick ] ] 1 | halt }
        if ($3 isnum) { 
          set %t1 $calc($3 - 32)
          set %temp $calc(%t1 * 0.55555555555555555555555555555556)
          msg $chan Converting $3 $+ $chr(176) Fahrenheit to Celsius: $round(%temp,2) $+ $chr(176) Celsius
          set -u5 %no.flood. [ $+ [ $nick ] ] 1
        }
      }
      if ($2 = c) {
        if ($3 = 0) { msg $chan Converting 0 $+ $chr(176) Celsius to Fahrenheit: 32 $+ $chr(176) Fahrenheit | set -u5 %no.flood. [ $+ [ $nick ] ] 1 | halt }
        if (!$3) { msg $chan $nick $+ , I need a temperature to convert. }
        if ($3 isletter) { msg $chan Converting $3 $+ : You\'re an Idiot? | set -u5 %no.flood. [ $+ [ $nick ] ] 1 | halt }
        if ($3 isnum) {
          set %t1 $calc($3 * 1.8)
          set %temp $calc(%t1 + 32)
          msg $chan Converting $3 $+ $chr(176) Celsius to Fahrenheit: $round(%temp,2) $+ $chr(176) Fahrenheit
          set -u5 %no.flood. [ $+ [ $nick ] ] 1
        }
      }
    }
  }
}

atleast this way it returns syntax when users perform the command improperly, and checks for flooding.