TOD calculation 3° slope 
Calculation of the descent rate 3° / 5% slope  Calculator  

OR OR From the indicated speed (IAS)  
IAS = Indicared AirSpeed >> speed directly indicated by the instruments, it comes directly from the external probes (Pitot), it is thus impacted by the density of the air mass. 
TAS = True Air Speed >> speed of an aircraft in relation to the air mass in which it is located, independently of its density. 
GS = Ground Speed >> speed relative to the ground. 
MACH = is the ratio of the speed of the aircraft in the air mass to the speed of sound in the same air mass. 
Ratio between the speeds: 
TAS=IAS+(FL/2) 
GS=TAS+Wind component 
MACH=TAS/600 
Formulas used : 
TOD distance = ( Cruise altitude (ft)  Target altitude(ft) / 𝟏𝟎𝟎 ) / 3° 
TOD distance = ( FL cruise FL target) / 3° 
VS (ft/min) = GS x 5 
VS (ft/min) = MACH x 3 x 3000 
The calculation of the SV from the MACH and IAS speeds takes into account the wind component The calculation of the VS from the IAS speed uses the formula TAS=IAS+(FL/2). The FL used is the intermediate FL between the cruise FL and the target FL. 
Source : https://mediawiki.ivao.aero/index.php?title=Top_of_descent_calculation 
Descent speed (source Eurocontrol)  
Aircraft code ICAO  Cruise >> FL 260  FL260 >> FL100 
A319/A19N//A320/A32N//A321/A21N  .78  290 
A332  .80  
A333/A338/A339  .81  
A342/A343/A345/A346  .81  
A388  .83  300 
B722  .72  280 
B731  .68  270 
B732/B733  .70  280 
B734/B735  .74  280 
B736/B738  .78  280 
B737  .76  280 
B742  .84  290 
B748  .85  250 
MD11  .83  300 
DH8D    270 
CL60  .75  250 
T154  .72  290 
AN24    240 