An e-flight calculator will get you in the ball-park, provided the input is correct.
E.g.
www.ecalc.ch (several languages)
Sticky:
e-flight calculators (compilation) - RCG
- To trust is good
- To calculate is better
Will get you to say within 10%, IF input ok.
e-flight calculators & propdata (compilation)
- To measure is a must
Without a watt-meter you are in the dark. Until something starts to glow 
A watt-meter will more than pay for itself, several times over, your battery, ESC and motor will love you for it. After calculating, always check current when you have a new/changed setup. Will also help you find optimal setup. And it's a great tool for debugging your power train.
- Keep battery-/watt-/multi-meter wires short
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds
- Hyperion Emeter II, with optical and electrical tach, servo tester, local&remote logging - RCG
A very long clearance sale, originally +200$, now literally peanuts
Vriendelijke groeten
en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
• E-flight calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •