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When you say 85dB you probably mean the loudspeakers sensitivity which is measured by feeding the loudspeaker with a voltage that produce 1W at its impedance and a microphone at a distance of one meter from the loudspeaker.
To produce 1W in a 8ohm speaker you need 2,83 volts RMS, same voltage into a 4ohm speaker will produce 2W, since we are dealing with power the measured value in dB/watts has to be lowered by 3 db=half to get it right or decrease the voltage to 2 volts RMS.
I don't think there is any formula that can tell you how much the loudspeaker can withstand in watts but you can measure its sensitivity, think the only way is to go by the manufacturers specifikation.
Hopefully this will sheed some ligh to your question

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It is a little 4 inch speaker that has a max continuous power rating of about 10W to 20W RMS.
If it gave 85dB at 1m with 1 Watt input then it is not sensitive.

Watts ratios can be calculated in dB:
3dB is half or double the power.
5dB is 0.316 or 3.16 times the power.
10dB is 1/10th or 10 times the power.
So 100W is 20dB louder than 1W. If the speaker could survive that much power then the loudness will be 105dB. I bet its voice coil will glow red hot at 100W just before it melts.
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So it means that the max power of the speaker is 100W and the RMS is about 20W, right?

No. The max continuous RMS power for little 4 inch speakers is only 10W or 20W.
What is a "max power" for a speaker? You must say for what duration. A speaker might be able to handle 100W for only a few milli-seconds, but might burn if it plays 20 Watts RMS continuously.
Continuous RMS power is the amount of real power.
"Peak" or "maximum" power that is advertised for a speaker or amplifier is simply the RMS number doubled. The peak voltage of a sine-wave is its RMS value times 1.414. Since the voltage is higher then the peak current is also higher. Power equals voltage times current so since both are increased by 1.414 then the power equals 1.414 times 1.414= 1.999 times more.
"Music power" is momentary peaks of power. "Peak music power" very high numbers are also advertised.

Since home speakers are usually 8 ohms and car speakers are usually 4 ohms, your speaker is probably for cars. Many car speakers and amplifiers have advertised power ratings that are very false. 
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Hi Audioguru,
This is an example of a 16,5cm car speaker (the same shop which i've bought my speakers from). As you said "A speaker might be able to handle 100W for only a few milli-seconds, ... " so in this case, this speaker might be able to handle 200W for a few milli sec.  ?
General specifications
Top power, IEC 268-5 (W)  220 
Max. power in, IEC 268-5 (W)  200 
Power out, IEC 268-5 (W)  45 
Frequency(Hz)  50 - 20 000 
Sensitivity (dB/W/m)  90

Thanks for being patient to answer my boring questions  ;D

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