A B737 MADE AN ENROUTE FUEL STOP. BECAUSE 40000 POUNDS OF PAYLOAD WAS NOT INCLUDED IN GROSS WT CALCULATIONS; THE ARRIVAL FUEL DECREASED TO THE EMERGENCY LEVEL.

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WE WERE IN ZZZ PREPARING TO DEPART FOR ZZZ1. WE WERE PERFORMING PREFLT DUTIES. FLT ATTENDANTS ASKED FOR BLANKETS. WE CHKED OUR SELCAL AND IT DID NOT WORK. WE HAD MAINT COME OUT TO THE ACFT; THEY CHKED IT AND CLRED IT FOR SVC. AGENT BROUGHT DOT Z AND FO INPUT NUMBERS INTO ACARS AND RECEIVED WT AND BAL INFO. WE GOT A CTR OF GRAVITY OK MESSAGE ALONG WITH A MESSAGE THAT SAID 10% OVER PLANNED TKOF WT. WE CHKED OUR MAX TKOF WT AND ACCEPTED NUMBERS. WE CLOSED THE DOOR AND DEPARTED FOR ZZZ1. WHILE CLBING TO CRUISE ALT WE SAW WE WERE TOO HVY FOR PLANNED CRUISE FL400 SO WE STOPPED AT FL380 FOR APPROX 10-15 MINS AND THEN CLBED TO FL400. I LOOKED AT ESTIMATED LNDG FUEL AND NOTED IT WAS QUITE A BIT LESS THAN PLANNED LNDG FUEL. SUPPLEMENTAL INFO FROM ACN 793422: AFTER LEVELING OFF AT A CRUISE STEP ALT OF 38000 FT; THE FLT CREW NOTICED THE ESTIMATED FOB UPON ARR AT ZZZ WAS SCHEDULED TO BE APPROX 5.2. AS THE FLT PROGRESSED THE ESTIMATED FOB AT ZZZ CONTINUED TO DROP 4.9 THEN 4.5; 4.2 AND EVENTUALLY DOWN TO 3.6 (EMER FUEL). WE THEN CLBED THE ACFT TO 40000 FT TO SEE IF THE LNDG FOB WOULD CHANGE. WE ALSO VARIED THE COST INDEX AND CRUISE SPDS TO SEE IF THIS WOULD ASSIST US. WE NOTICED DURING CLBOUT AND INITIAL LEVELOFF; THE WINDS WERE APPROX 30 KTS FASTER THAN FORECAST PER OUR FLT PLAN. AS THE FLT PROGRESSED; THE WINDS ALOFT BECAME MORE ACCURATE (WITHIN 5 KTS). IT WAS AT THIS TIME WE CAUGHT THE ERROR. OUR PAYLOAD WAS INCORRECTLY PLANNED AND WE WERE MISSING PAYLOAD OF 137 PAX AND APPROX 4000 LBS OF CARGO. ROUGHLY 40000 LBS OF MISSING PAYLOAD WHICH DRASTICALLY CHANGE OUR ENRTE FUEL BURN. PAYLOAD ON OUR FLT PLAN SHOWED 616 LBS. SEVERAL MESSAGES WERE SENT VIA ACARS AND WE INFORMED OUR DISPATCHER OF THE ERROR AND TOLD HIM WE NEED A FUEL STOP PRIOR TO REACHING ZZZ. WE DIVERTED THE FLT TO ZZZ1 FOR FUEL. AS A SIDE NOTE; THE FLT PLAN FOR THE CONTINUATION OF FLT X TO ZZZ FROM ZZZ1 WAS INCORRECTLY PLANNED BY THE OMISSION OF THE AFOREMENTIONED PAYLOAD ERRORS A SECOND TIME. THE CAPT THEN PHONE DISPATCH TO AMEND OUR FUEL BURN AND FOB PRIOR TO LEAVING ZZZ1. POSSIBLE REASONS; DISTRS INCLUDE: SCHEDULE CONSTRAINT; TOO MANY PEOPLE IN THE COCKPIT PRIOR TO DEP (MAINT AND GATE AGENT); RAMP AGENTS REQUESTING PERMISSION TO DISCONNECT AIR -- ELECTRICS SIMULTANEOUSLY AWAITING FOR 'NUMBERS.' RELIANCE ON A PROPER FLT PLAN.