What speed and current occur at this load?

A brushed DC motor at a fixed voltage has a torque-speed relationship that is close to a straight line. At no load it runs at its highest speed and draws only the current needed to overcome friction and brush drop. Load it and the speed falls; keep loading it and the speed reaches zero at stall, where the torque is highest and the current is the applied voltage divided by the winding resistance. Because it is close to a line, the whole characteristic can be reconstructed from two published points, which is exactly what the catalog gives you.

Current is the other line. It rises from the no-load value to the stall value in proportion to torque, because torque in a permanent-magnet DC motor is proportional to armature current through the torque constant. That constant, in mN·m per amp, is the stall torque divided by the current above no load, and it is the single most useful number to carry into a driver design: it converts a torque requirement straight into a current requirement.

Both lines scale with voltage. Halve the supply and the no-load speed halves, the stall torque halves, and the stall current halves, because the winding resistance has not changed. That is why the tool will scale a published 12 V winding to 6 V, and why it marks the answer as extrapolated when it does. It is a defensible first approximation and it is not measured data.

Where the line stops being true matters more than where it is true. Near no load, friction and iron loss are a large fraction of a small torque, so real motors run slower than the line predicts. Near stall, the winding heats within seconds, resistance rises, and the real stall torque falls below the cold catalog value; a hot motor makes less torque and draws less current than the number on this page. Brush voltage drop, roughly constant, eats a larger share of a low supply voltage, so the model degrades below about 40% of rated. And the peak-power point sits at exactly half of stall torque in this model, which is a useful landmark and a terrible place to run: efficiency there is under 50%, and everything else is heat.

The best-efficiency point is much closer to no load, typically at 10 to 20% of stall torque, which is where the rated point of a well-specified winding usually sits. If a calculated operating point is far from the rated point, the winding is probably wrong for the application even when the arithmetic works.

One thing this page cannot do, at all, is tell you how long the motor lasts. Brush wear, commutation, bearing life and grease life are functions of speed, current density, spark energy, temperature, orientation and contamination. None of them are recoverable from a torque-speed line. A point that looks comfortable here can still be a short life, and a point that looks marginal can run for years. Life comes from test evidence.