Formulae for calculating resistor for LED in parallel

Formula to calculate LED in parallel always confused me, i.e.

I have three Led's with forward drop of 1.7 V and 10 milliamp current. I am using 9 V battery with 15 miliamp. I used this website that gave me value of resistor to be used when LED in series or in parallel.

http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

For 3 led in Series, value is 390 ohms, i.e.
9V-(1.7*3)V/0.01= 390 Ohms

For 3 LED in 'Parallel', it states voltage drop of 243.33 Ohms ??
Can anyone please advise me on what formula i need to use to LED in parallel to derive this value?

I know ampere gets added in parallel...hence i tried 9/(0.01*3)=300 ohms and not 243.33 OHMS..So where I am going wrong..plz advise?

 

led's in parallel (without each having its own resistor) can be highly problematic and is generally frowned upon. Each led (or series string) should have its own current limiting resistor.

And "current" is divided in parallel. So if you want each of the 3 led's getting 10mA your supply needs to put out 30mA.

9V-1.7V = 7.3volts
7.3V/.03amps = 243.3 ohms

 

Thanks you guys- i ran the led in series and with parallel i had 330 ohm with every led. So it worked fine.

Again thanks for explaining how the resistance was calculated and advise