Blood Oxygen (SPO2) Calculator
Use our SPO2 calculator to find out what your blood oxygen readings mean.
Enter your the reading from your pulse oximeter into the box.
What does your blood oxygen level mean?
We all know that we need oxygen to survive. But breathing air in is only the first step. The oxygen that we breathe in then gets absorbed through the lining of our lungs into our bloodstream. There, it gets picked up in our blood and carried by a molecule called Haemoglobin (medical abbreviation Hb), which takes it to the cells in our body where it is needed and is used to keep the cells working. Your blood oxygen level gives you an idea of how much oxygen there is in your bloodstream at any one point in time.
What is a normal blood oxygen level?
The quick and simplified answer here is 96% or higher. Meaning that of all of the Haemoglobin travelling through the arteries in your finger (and thereby, probably throughout the rest of the body), 96% or more has oxygen attached.
Is below 96% a bad blood oxygen level?
However, this isn’t always the final or best answer. A normal blood oxygen can vary depending on any medical conditions that you may have (especially when it comes to chronic lung diseases or other conditions that affect your Haemoglobin, like anaemia), and even on your altitude. One study found that the average oxygen level for people who lived in the very elevated city of La Rinconada, Peru (at an altitude of 5100m) was only 81%5.
Similarly to how low levels may potentially be normal for some people, high levels could confusingly still be abnormal! This is why it’s important to always assess for a change or drop in your normal levels as that may signify an issue, especially for people with black or brown skin. For instance, if you are always around 99 or 100% and your levels have dropped to 96%, especially if you have any other symptoms to suggest an infection or illness, it would be wise to seek medical attention. You can do this by contacting your NHS GP or ringing 111.
How is your blood oxygen level (SpO2) measured?
How does a pulse oximeter work?
One of the easiest ways to do this is by using a pulse oximeter1. This is a small device that pops on to a finger. By shining lights through the nail (a process that you can’t actually feel and doesn’t hurt at all), it measures how much of the Haemoglobin in the blood in your finger has oxygen attached to it. It gives the result as a percentage, with 100% meaning all of the Haemoglobin coming through the area has oxygen attached.
Advantages of using a pulse oximeter
There are lots of advantages to using a pulse oximeter compared to other ways of testing oxygen levels – it isn’t invasive, it doesn’t hurt, you don’t need other equipment to process the reading, and the devices are relatively inexpensive and widely available. For this reason, they are used by clinical staff in all settings both in the hospital and in the community, as well as by many people in their own homes.
Disadvantages of using a pulse oximeter
However, there are potentials for inaccuracies. For instance, we know that the readings of all pulse oximeters around the world are potentially affected by nail varnish, cold fingers and pigmented skin. This is why the NHS advises that, for people of colour using pulse oximeters to monitor a Covid-19 infection, it is important to monitor for a change in your normal reading. This can be significant, even if the number itself is still considered to be in the normal range2. And of course, it is helpful to remove nail varnish and warm up any cold hands before using a device.
Pulse oximeter alternatives
While pulse oximeters are still very useful and appropriately the first line in assessing oxygen levels in the vast majority of cases, another option is by taking a blood sample from an artery (usually the wrist is used) and analysing this with a special machine. This is known as an Arterial Blood Gas, or ABG. This can be quite painful and is typically only done in more extreme circumstances, for instance if someone is unwell in the Emergency Department. This is the most accurate way of knowing the exact oxygen levels in the blood and also gives other markers of how unwell someone may be, such as a lactate level.
A third option for measuring oxygen levels is checking a Capillary Blood Gas, or CBG. This is when blood is collected from the ear, often using a pinprick. This can be useful when someone’s oxygen levels are low and increased accuracy is needed compared to using a pulse oximeter. For instance, it is often helpful for people who suffer from chronic lung disease, who may need repeated assessments of oxygen levels.
Can you take a pulse oximeter reading at home?
Absolutely! In fact, with how much of medicine is becoming digitalised and remote these days, there are lots of advantages to monitoring your oxygen levels at home, especially if you are battling an infection. We know that in Covid-19 infections in particular, oxygen levels can drop even before someone has symptoms of feeling more short of breath3. By monitoring oxygen levels at these times, you can then be equipped to be proactive about getting important medical attention sooner than you might do otherwise. This might help prevent an illness from getting more serious by getting the treatment you need in time.
Disadvantages of measuring your blood oxygen levels (SpO2) at home?
Of course, it is equally important to always recognise limitations of any device that we use and the potential for normal numbers to falsely reassure us. For that reason, you should pay attention to any symptoms you are experiencing and take them seriously. If you are concerned about any symptoms, don’t take a normal oxygen reading as complete reassurance that there isn’t a significant issue present. Ensure that you listen to your body and it’s prompts and seek medical attention appropriately. For more information about interpreting the results from a pulse oximeter during times of illness, please see the NHS guide here4.
Causes of low blood oxygen
There are lots of potential causes of a low reading on a pulse oximeter. These include:
- Chest infection
- An infection elsewhere that is damaging the body (sepsis)
- Asthma attack
- Blood clot on the lungs
- Chronic lung disease (e.g., bronchitis or emphysema)
- Other lung abnormality such as accidentally inhaling something, fluid in the lungs (pulmonary oedema), fluid in the membrane around the outside of the lungs (pleural effusion)
- Heart attack or heart failure
- High altitude
- Obstructive sleep apnoea
- Artificially low readings due to cold fingers, nail varnish, incorrect oximeter size for age, etc.
Why else could my blood oxygen (SpO2) level be low?
Other instances where the actual oxygen levels in the blood may be low but a normal reading is showing in a pulse oximeter include:
- If you remember, the pulse oximeter gives a reading as a total percentage of the Haemoglobin that has oxygen attached. Anaemia, or ‘low blood count’, is the medical term for having less Haemoglobin than normal. As an analogy, picture Haemoglobin as an Uber, oxygen as the passenger inside, and the town represents a whole body. Say Town A has 100 Ubers, and 98 of them are carrying passengers. Town B has only 10 Ubers. If all 10 of them are full, their ‘SpO2’ is 100% but they are carrying way fewer people around than Town A’s. It’s the same issue with someone with anaemia – their reading may look great as proportionally lots of their Haemoglobin has oxygen attached, even though the actual amount of oxygen they are carrying may be quite low.
- Carbon monoxide poisoning and heavy smoking can both increase the amount of a type of Haemoglobin in the blood that doesn’t actually carry oxygen, called carboxyhaemoglobin, or COHb. Think of it like an Uber driver who for some reason refuses to take passengers. Due to the mechanics of how all pulse oximeters work, these non-functioning molecules sneak past undetected or even by posing as hard-working ones. This essentially leads to artificially high readings that don’t reflect the true, lower oxygen levels present.
Effects of low blood oxygen
These can be split into short term effects and long term.
Short term effects (symptoms) of low blood oxygen levels
Short Term Effects include tissue damage, with by-products of damage like lactic acid building up (also known as lactate and tested for with an ABG as mentioned earlier). Eventually with prolonged dangerously low oxygen levels, whole organs can shut down and death can occur. Symptoms of low blood oxygen include feeling short of breath, lightheaded, and/or confused. The heart might pump harder and faster to try to push more oxygen round, which can cause feelings of palpitations or objectively a raised blood pressure or faster heart rate.
Long term effects (symptoms) of low blood oxygen levels
Long Term Effects: Provided the oxygen levels are not dangerously low, they can trigger the body to make more red blood cells (these are the cells that house Haemoglobin in the blood). This explains why athletes for endurance sports will often train at higher altitudes: the demands they put on their body while having less oxygen activates the feedback loop for increased production of red blood cells (and Haemoglobin). When they then return to lower elevations, their higher blood count can help boost their performance. Chronic lung disease does a similar thing although people with chronically low oxygen related to a lung condition can be in danger of having blood that is too thick with these red blood cells (polycythaemia), which can in itself cause problems.
How to increase your blood oxygen level
If your blood oxygen level is newly low, please seek medical attention as appropriate as this is not normal and can be a sign of something serious. Ring your NHS GP or 111 for mild symptoms or attend A&E or ring 999 for life-threatening symptoms.
If you normally have a low blood oxygen level and you’d like to increase it, then really this is done by treating the underlying cause. For someone with sleep apnoea, this would involve using a breathing (CPAP) machine at night. For others, it may mean ensuring good compliance with and optimisation of medication.
The main lifestyle change that can boost oxygen levels is quitting smoking. Even for people with severe lung disease, it is never too late to quit – this can still have a positive effect on your health. By quitting, you not only start to protect the lungs from the toxic chemicals, you also start to get rid of that non-functional Haemoglobin that doesn’t carry oxygen, boosting efficiencies all round.
- Greenhalgh T, Knight M, Inada-Kim M, Fulop N J, Leach J, Vindrola-Padros C et al. Remote management of covid-19 using home pulse oximetry and virtual ward support BMJ 2021; 372 :n677 https://doi.org/10.1136/bmj.n677
- Rojas-Camayo J, Mejia CR, Callacondo D, et alReference values for oxygen saturation from sea level to the highest human habitation in the Andes in acclimatised persons. Thorax 2018;73:776-778. https://thorax.bmj.com/content/thoraxjnl/73/8/776.full.pdf
- Glass KL, Dillard TA, Phillips YY, Torrington KG, Thompson JC. Pulse oximetry correction for smoking exposure. Mil Med. 1996 May;161(5):273-6. PMID: 8855058. https://pubmed.ncbi.nlm.nih.gov/8855058/
- Hampson NB. Pulse oximetry in severe carbon monoxide poisoning. Chest. 1998 Oct;114(4):1036-41. doi: 10.1378/chest.114.4.1036. PMID: 9792574. https://pubmed.ncbi.nlm.nih.gov/9792574/