How to calculate the needed voltage and current to supply for Arduino I have Arduino MEGA and I'm connecting to it 5 servomotors.

{ 3 TowerPro M995 
Basic Information

Modulation:  Digital  
Torque:  4.8V: 130.5 oz-in (9.40 kg-cm)
6.0V: 152.8 oz-in (11.00 kg-cm)

Speed:  4.8V: 0.20 sec/60°
6.0V: 0.16 sec/60°

Weight:  1.94 oz (55.0 g)  
Dimensions:  
Length:
1.60 in (40.7 mm)

Width:
0.78 in (19.7 mm)

Height:
1.69 in (42.9 mm)  
Motor Type:  3-pole  
Gear Type:  Metal  


2 Towerpro M996R 

Basic Information

Modulation:  Digital  
Torque:  4.8V: 130.5 oz-in (9.40 kg-cm)
6.0V: 152.8 oz-in (11.00 kg-cm)

Speed:  4.8V: 0.19 sec/60°
6.0V: 0.15 sec/60°

Weight:  1.94 oz (55.0 g)  
Dimensions:  
Length:
1.60 in (40.7 mm)

Width:
0.78 in (19.7 mm)

Height:
1.69 in (42.9 mm)  
Motor Type:  3-pole  



}  


So how can I calculate the voltage and current needed to supply this Arduino?
Voltage is simple - a device has a requirement and that is what you have to provide. In the case of the servos that is a range of voltages that it can use. So 5V is a good voltage.  The Arduino can take 5V if you feed it in to the 5V pin or the USB port, or 7-12V if you feed it in to the barrel jack.

The current you cannot calculate. You haven't provided enough information. The calculation, when you have the information, is quite simple: for all the devices on the same power supply add the currents requirements together. The supply has to be able to provide at least that amount of current.