Help Calculating the resistance required.

so, i've got arduino powered 7v battery. have jumper wire going 'gnd' digital pin through resistance , speaker of 8Ω , 0.25w.

the equation i'm using calculating resistance:

(source volts - led volts) / (current / 1000) = resistance

for speaker of 0.25 w , 8 ohms 176.776695 ma , 1.41421356 v.

(source volts - speaker volts) / (current / 1000) = resistance
(7v - 1.414v) / (176.78 / 1000) = 31.6 Ω.

did right? have connect 31.6 Ω resister jumper wire, solder speaker wire's end (cathode); take other end of wire (anode) , insert in whichever digital pin want?

sorry if this  noob question, i'm new this.

do other way.
max current arduino pin can supply 40ma.
if work out 5volt (5/0.04), 125ohm.
the internal pin resistance ~40ohm, external 85ohms.
subtract 8ohms (speaker) = 77ohms.
this value kill arduino......

better work 20ma.  >>> 202ohm.
the safe/practical value 220ohms.
a value see pop many times.

led calculations bit different.
you have know leds forward voltage, , drive current.
leo..

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