What is the voltage a human body could withstand under normal conditions?

It's not the voltage that kills...it's the amperage.

Watts is what you are looking for?
It's not the voltage you need to worry about -- it's the current. Current is measured in amps, and it's the measure of how many electrons pass a given point in one second (so it's the actual "movement" of the electricity). A car battery is only 12 volts but can have well over 50,000 cranking amps, and that could easily kill you. On the flipside, I have been shocked putting fluorescent tubes in light fixtures that are on 120 volt circuits and it didn't kill me -- but it sure gave me a nasty jolt!
Any amount - voltage does not hurt you in the least by itself, it is the current cause by a voltage that hurts you. A fatal current usually ranges between 100 and 200 milliAmperes. The relationship is voltage = current * resistance. If there is alot of resistance to your body (dry skin, large distance between voltage contacts, not grounded well), tens of thousands of volts would not hurt you in the least. In certain situations where there was very little resistance from a human body (wet skin, contacts close together, very well grounded) then voltage as low as 30-40 volts has been fatal.
It's not the voltage, it's the current.

A general rule of thumb is anything even close to 1 Amp is very, very bad.
I believe I've heard that only a few milliamps, measured at the heart, can screw you up.

Also depends on what part of the body you get the electricity to flow through. And whether the current travels only on the skin, or actually through the organs inside.

Heard a story (urban legend?) of a dumb guy who nearly killed himself by trying to test his body's internal electrical resistance with a simple voltmeter. He kind of didn't remember that a voltmeter checks voltage by sending out a bit of electric current, usually supplied by a simple battery, which is normally harmless, BUT

he jammed the probes into his thumbs, giving the current direct access through his wet, slimy, conductive insides, from one arm, through the heart and out the other arm. A small but deadly zap.
Well, define withstand. During electronics class in high school, we took the variable power supply and slowly turned up the voltage as we held one wire in each hand. Smart, huh? Well, we found that it started tingling at about 30V. Above 50 started to get really zingy.

I found from personal experience that 120V can give you a really good jolt. My brother found that contacting 220V holding a pair of bare metal needle nose pliers in each hand will hurt. It hit really hard and made his chest sore for a week.

The danger comes when the current flows through your heart. Or brain too. People have survived extreme accidents with electricity, ending up severely burned and disfigured. But you can be killed by electricity and not show any outward signs if it gets you just right.
That depends on many factors. What usually kills people during electric shocks is the current. It varies from one person to another, but in general 30mA ac cross the heart is considered potentially fatal. In order to calculate how much voltage is required to get 30mA across the heart you would have to know the total resistance for the entire electric path. Then you could use Ohms law (Voltage = Currrent X Resistance) to get your answer. Keep in mind that the resistance changes once you reach the breakdown voltage of the tissue. Water content will also play a big factor in the resistance. And finally, this is assuming that the voltage is DC.
Even if you wanted amps, not the voltage, it would completely depend on how and where the current was placed (and how long). A current passing through your hand has a different effect than one passing through your brain or heart.

If you're completing a circuit with your body, its best to do it in places where a temporary reset of the body's own electrical system isn't likely to kill you...remember that *most* of the current is going to take the most direct path.

Not that I recommend completing a circuit with your body.
I agree with pickay... 100%
I tend to not like the saying "it is not the voltage that kills, it is the current". I usually say, it depends on your body's resistance at the time you close a circuit and which body parts have current flowing through them.

For example, the most dangerous current is that which flows through your chest (where your heart is located). Other dangerous places on your body are your midsection and head (where high currents greater than 1A could burn/damage vital organs or the brain).

BUT, let's assume for your question that you are holding a hot wire and neutral wire in each hand (quite dangerous since current will likely flow through the heart). Ventricular fibrillation can occur anywhere from 35mA for children up to 100mA for men (depends on body size). So, if your resistance is very low (eg: your hands are saturated wet or the electrodes are piercing the skin) it may not take all that much voltage to induce ventricular fibrillation (fatal in minutes if no defibrillator is nearby).

Long story short, read the link given below. I agree with that info for the most part. They calculate that voltage as low as 340V could kill you if your hands have even a little bit of moisture on them. I would say don't go touching 120 or 240 V supplies; it hurts a LOT and could kill you under special circumstances (rare, but it can happen). You pose a good question, which unfortunately has a fairly complicated answer.
They say its not the volts its the amps that kills but that is not true you can be killed by any voltage it just depends on how you are grounded and the resestance of your body and the path it takes through your body and the length of hookup time you are exposed.Usualll you are ok if the voltage can not produce over 50milliamps of current anything over that is very uncomftorable.

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