B12 Dosage Estimator

Many factors affect b12 absorption1,4 such as gastro intestinal issues, alcohol consumption, genetics and how you take b12 (oral, sublingually, injection). Use this as a rough guide only to see if you're way out of whack with what you're taking now and see your doctor if you are concerned. See bottom of page for source material
NOTE: Overlapping (saturation) doses are not considered. See below for more information
NOTE: Err on the side of caution as there is currently no evidence to suggest large doses are detrimental

Comments & corrections welcomed
Age Dosage (mcg) Frequency (days) Avg Daily / RDI Amount of dose Absorbed
The chart below describes the absorption rates of b12 based on the referenced data at the bottom of the page. As you can see b12 absorption drops off drastically after 1mcg and then tends towards about 1% of the dose. In simple terms the receptors that absorb b12 get saturated at ~1.5mcg2 so any (oral) bigger than that passes along the gut and is absorbed (if at all) by passive diffusion until the receptors are ready to accept more (about 4-6 hours)3. Sublingual absorption is different and I have yet to find much comparison data except that which suggests they are similar in correcting deficiencies5.

References:

Absorption:
1, 2The American Journal of Clinical Nutrition, Volume 89, Issue 2, 1 February 2009, Pages 693S–696S
3O’Leary F, Samman S. Vitamin B12 in Health and Disease. Nutrients. 2010;2(3):299-316. doi:10.3390/nu2030299.
4Effects of moderate alcohol consumption on folate and vitamin B12 status in postmenopausal women: European Journal of Clinical Nutrition volume 58, pages 1518–1524 (2004) doi:10.1038/sj.ejcn.1602002
RDI/Dosages
Nutrient Reference Values for Australia and New Zealand - Ministsry of Health
National Institutes of Health B12 RDI table
VITAMIN B12 INCREASED DOSE: 300-500 MCG - Absorption estimation formula
Method
I have yet to find compelling evidence of whether oral or sublingual is better (Indications are they are very similar but I'm still looking) but here are some articles worth exploring if you are interested.
5Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route
Kolber MR, Houle SKD. Oral vitamin B12: a cost-effective alternative. Canadian Family Physician. 2014;60(2):111-112.
SUBLINGUAL VITAMIN B12
Sublingual Mucosa As A Route For Systemic Drug Delivery. Shri Baba Mastnath Institute of Pharmaceutical Sciences and Research, Asthal Bohar, Rohtak ­124001 , Institute of Pharmaceutical Sciences, Kurukshetra University, Kurukshetra­ 136119 (Haryana), India