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Organic molecules control calcite growth and crystal morphology, influence biomineralization processes, and offer clues for optimizing antiscalants for industry. Here we quantified the effect of amino acid monomers, aspartic acid (Asp1), and glycine (Gly1), and their polymers (Aspn, Asp5, and Gly5), on calcite growth rate, in a constant composition setup. Asp1 and its polymers inhibit growth, with rate decreasing as amino acid chain length increases. For 2 mM Asp1, fractional inhibition (FI, where 1 represents complete inhibition) was 0.54; for 0.0012 mM Aspn, FI = 0.94. Gly1 and Gly5 only marginally affect growth (−0.1 < FI < 0.1); indeed, they slightly promote growth at most tested concentrations. Fitting of adsorption isotherms (Langmuir, Langmuir–Freundlich, Flory–Huggins) confirmed that Asp polymers adsorb strongly, explaining their strong control on calcite growth, but Gly1 and Asp1 adsorb less due to competition with carbonate ions. ΔG ads (Aspn) = −39 kJ/mol; ΔG ads (Asp5) = −50 kJ/mol; ΔG ads (Asp1) = −21 kJ/mol; and ΔG ads (Gly1) = −22 kJ/mol. The morphology was equally affected. Crystal edges became rougher, and corners, more rounded. Overall, the number of carboxyl groups and length of the carbon chain correlated with the lowest growth rate.