Blood Oxygen Drop Calculator (Today vs Your Usual SpO2)

A pulse oximeter number means most next to your own usual one. A healthy 30-year-old who always reads 99% and reads 95% today is still inside the normal range, and still four points down. Someone with lung disease whose usual is 91% is not in trouble at 91%, whatever a chart says.
This page reads today’s number both ways: against the general scale, and against your usual reading or the target your doctor gave you.

How big a drop matters

Oximeters are not precise instruments. The FDA’s accuracy standard for a cleared fingertip oximeter allows a typical error of up to 3 points against a blood test, many cheap ones are not cleared at all, and two readings a minute apart on the same finger often differ by 1 or 2.
So a 2-point difference from your usual is noise, and 3 is at the edge of it. A drop of 4 or more, measured the same way at rest, is bigger than the device can explain, and it is worth acting on even while the number still looks normal.

“The same way” carries a lot of that sentence: same finger, warm hand, no nail polish, five minutes sitting still, and the number read once it stops changing. Compare a morning reading with a morning reading.

The scale it uses

That is the sea-level scale of the blood oxygen guide. If you have travelled to altitude since your usual reading was taken, use that page instead, because the whole scale moves with height.

If your doctor gave you a target of 88 to 92%

People with COPD (chronic obstructive pulmonary disease) and some other conditions can hold on to carbon dioxide when they are given too much oxygen. So the British Thoracic Society’s guideline aims their saturation at 88 to 92%, where most people are aimed at 94 to 98%.
Pick that option and the page judges you against your target instead. Above 92% is fine on room air; on oxygen, your plan may say to turn it down. Under 88% is below your target, and the action plan you were given says what to do next. Under 85% is an emergency either way.

What it cannot tell you

A normal number does not rule out illness. Working hard to breathe in order to keep the number up is a warning sign of its own, which is why the heart rate box is there: a falling saturation with a rising pulse means the body is straining to hold the line.
And an oximeter reads falsely normal in carbon monoxide poisoning, because it cannot tell carbon monoxide on the hemoglobin from oxygen. To see what a reading means for the oxygen pressure in your blood, try the SpO2 to PaO2 calculator.