How hot would mars' climate be if it had an atmosphere close to earth's?

"How hot would mars' climate be if it had an atmosphere close to earth's?"

If our Moon's temperature swings are compared to Phobos:

Phobos −4 °C to −112 °C (average is -58°C)

Moon -173 °C to 127 °C (average is -23 °C)

Then Mars should be on average 25 C° cooler than Earth (Earth average is 10 to 20 °C).

Our atmosphere is increasing the average surface temperature by around 70 degrees C. It's not just the insulating effect that Earth's atmosphere is so useful for, it's the distribution of heat over the day and night cycle so life isn't subjected to a fatal 200 degrees C temperature swing each evening!

Mars' thin atmosphere does help to mitigate the temperature swing a little and does retain some heat, but an average day is something like 80 degrees C cooler than any on Earth.

Were you to magically grant Mars an Earth-like atmosphere the first thing it would probably do is set about losing it as there is no magnetic field to protect it from the scouring effect of incoming radiation. Hydrogen would be continually split off from oxygen in any water molecules within the extensive Martian atmosphere (the low gravity means that the atmosphere would extend much higher up into space than it does on Earth leading to a lower pressure at ground level). This leaves a hydrogen deficit (and therefore water deficit) over time and a build up of free radical oxygen and ozone, both of which are toxic. Any terraforming effort on Mars in the future would need to be a permanent industry fighting daily against nature, finding ways of replenishing water (probably by bringing in asteroid and comet materials for processing) and scrubbing out ionising nasties. It'd be a monumental task which would probably verge on too much work to be worth it... unless our future civilisations are so advanced as to somehow make such projects a walk in the park.

In the short term the equatorial regions of Mars might become almost pleasant in a Siberian steppes kind of way. Warm enough for t-shirts during the summer at least. (not that I recommend it as you will be certain to develop skin cancer on Mars) Water would return to the surface and all the carbon dioxide would boil off the poles as their average temperatures climbed too high for the CO2 ices to remain locked. This would be good as the CO2 would act as a greenhouse gas as well as being useful for any plants you wished to attempt to grow there. Most of Mars' surface however will probably remain frigid inhospitable desert without significant geoengineering.

There would still be no fix for Mars' lack of a magnetosphere and the Martian temperature extremes caused by it's more pronounced elliptical orbit, so just giving it an Earth like atmosphere isn't enough to make it a good place to live. Insects have lifespans short enough to shrug off the worst of the radioactivity and some plants would probably thrive but the genetic mutation rate would be an order of magnitude higher than that seen on Earth. When you stress life in a major way, it causes evolution to cook on high heat and we might start to see some fascinating adaptations in only a few generations of time. I actually think the future species grown on Mars will grow ever more radiation hardened. I have no idea how they will do that but nature is extremely tenacious in finding ways to survive. We can't do that though so it's not an environment we will ever be safe in unless we want to be riddled with cancers. Maybe our descendants would grow clever enough not to change Mars to suit us, but to genetically alter ourselves to make us suitable for Mars... in the long run that may be easier.

Over a few million years your Earth-like atmosphere will begin to deplete, pressures will drop, water will become scarce or permanently frozen and the life will all die off again. Mars will return back to it's miserable dessicated current state.

About like Antarctica = 20 f = 11 c warmer than present average Mars' temperature.

That would really depend on the composition and density of that hypothetical atmosphere, wouldn't it?

In other words, your question has insufficient parameters to answer intelligently.

Temperate to cold. At best, it would be as warm as northern Europe.