We introduce a certain birational invariant of a polarized algebraic variety and use that to obtain upper bounds for the counting functions of rational points on algebraic varieties. Using our theorem, we obtain new upper bounds of Manin type for 28 deformation types of smooth Fano 3-folds of Picard rank ≥ 2 following Mori-Mukai's classification. We also find new upper bounds for polarized K3 surfaces S of Picard rank 1 using Bayer-Macrì's result on the nef cone of the Hilbert scheme of two points on S.where Eff 1 (X) is the cone of pseudo-effective divisors on X. Here is the statement of weak Manin's conjecture:Conjecture 1.1 (Weak Manin's conjecture/Linear growth conjecture). Let X be a geometrically uniruled smooth projective variety defined over a number field k and let L be a big and nef divisor on X. Then there exists a non-empty Zariski open subset U ⊂ X such that for any ǫ > 0 N(U, L, T ) = O ǫ (T a(X,L)+ǫ ).Note that for L = −K X , we have a(X, L) = 1 so that it backs up the word "linear growth".Remark 1.2. There are counterexamples to a version of this conjecture where one assumes L to be only big but not nef. See [LST18, Section 5.1]