If i don't know if my vitamin d levels are low and there's disagreement about what low looks like in any case, what would a safe dose be in case it is low

The short version: if you are healthy and have no symptoms, the safest dose is no supplement at all, because the evidence does not support blanket supplementation for people without deficiency.

The disagreement over what "low" means is real. The Endocrine Society says 30 ng/mL is the lower bound; the Institute of Medicine says 20 ng/mL is enough for bone health in most people [1]. That gap means a person at 25 ng/mL gets labeled deficient by one authority and normal by the other. The test is the only way to know where you stand, and even then, a single reading is a snapshot. Levels drop in winter; a summer 32 ng/mL can fall to 22 ng/mL by February [1].

If you decide to supplement without testing, the dose that is safe for nearly everyone is 400 to 800 IU per day. That is the range used in fortification and in most multivitamins, and it has never been shown to cause harm in healthy adults [1]. It will not fix a true deficiency quickly, but it will not hurt you either. Doses above 2000 IU per day are where the evidence gets thinner. The upper tolerable intake set by the IOM is 4000 IU per day, but that number is not a target; it is a ceiling [1]. Exceeding it raises the risk of hypercalcemia, which can cause kidney stones and heart rhythm problems.

The one hard rule: do not take high doses, 5000 IU or more, without a blood test. The reason is not fear of toxicity at ordinary doses. It is that the evidence for benefit from high-dose supplementation in people without documented deficiency is weak. A 2023 meta-analysis of 19 trials found no reduction in fatigue from vitamin D in people without deficiency [1]. If you are not deficient, the supplement is doing nothing except making expensive urine.

My call: if you are healthy and symptom-free, skip the supplement or take 400 to 800 IU daily. If you have symptoms or risk factors, get the 25-hydroxyvitamin D test first and dose against the result. Confidence: high that 400 to 800 IU is safe for everyone; moderate that it does anything for you if you are not deficient.