Press F3
Just input all the Info requested (read the note at bottom of dialog)

Click Button.

NOTE: IN RUNECSAPE COMBAT IS ALWAYS ROUNDED DOWN SO IT IT SAYS 77.98 YOUR STILL 77 ;)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;   RS Combat Calculator   ;
;Created by Rayth/LucSatise;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;Combat Formula Found on RS Forum
dialog RSLevels {
  ;Title and Options
  title "Runescape Combat Calculator"
  option dbu
  size -1 -1 200 130 
  ;Get Combat Skills Skills
  Text "Attack:", 3, 20 25 20 10
  Edit "", 4, 45 23 10 10
  Text "Strength:", 5, 70 25 25 10
  Edit "", 6, 95 23 10 10
  Text "Defence:", 7, 120 25 25 10
  Edit "", 8, 145 23 10 10
  Text "HitPoints:", 9, 20 40 25 10
  Edit "", 10, 45 38 10 10
  Text "Ranged:", 11, 70 40 25 10
  Edit "", 12, 95 38 10 10
  Text "Magic:", 13, 120 40 25 10
  Edit "", 14, 145 38 10 10
  Text "Summoning:", 15, 15 55 30 10
  Edit "", 16, 45 53 10 10
  Text "Prayer:", 17, 70 55 25 10
  Edit "", 18, 95 53 10 10
  Text "Combat Level:", 19, 120 55 40 10
  Text "", 20, 160 55 30 10
  Button "Calculate", 21, 80 70 40 20
  Text "* If Summoning/Ranged/prayer/magic level is odd decrease it by 1 when putting into here.", 22, 10 100 180 20  
}
on *:Dialog:RSLevels:SCLICK:21:{
  CalculateCombat
  did -ra RSLevels 20 %Combat
}
alias CalculateCombat {
  set %BaseLevel $calc((($did(RSLevels,8).text * 100) + ($did(RSLevels,10).text * 100) + ($did(RSLevels,18).text * 50) + ($did(RSLevels,16).text * 50)) / 400)
  set %Melee $calc((($did(RSLevels,4).text * 130) + ($did(RSLevels,6).text * 130)) / 400)  
  set %Ranged $calc(($did(RSLevels,12).text * 195) / 400)
  set %Mager $calc(($did(RSLevels,14).text * 195) / 400)
  if (%Mager > %Ranged) && (%Mager > %Melee) { set %Combat $calc(%Mager + %BaseLevel) }
  elseif (%Ranged > %Mager) && (%Ranged > %Melee) { set %Combat $calc(%Ranged + %BaseLevel) }
  elseif (%Melee > %Ranged) && (%Melee > %Mager) { set %Combat $calc(%Melee + %BaseLevel) }
}

alias f3 { dialog -dm RSLevels RSLevels }