Loewner driving functions encode simple curves in 2-dimensional simply connected domains by real-valued functions. We prove that the Loewner driving function of a C 1,β curve (differentiable parametrization with β-Hölder continuous derivative) is in the class C 1,β−1/2 if 1/2 < β ≤ 1, and in the class C 0,β+1/2 if 0 ≤ β ≤ 1/2. This is the converse of a result of Carto Wong [27] and is optimal. We also introduce the Loewner energy of a rooted planar loop and use our regularity result to show the independence of this energy from the basepoint. Recently, in [6] and [23] the chordal Loewner energy ∞ 0Ẇ (t) 2 /2 dt of η was introduced independently, and basic properties (such as rectifiability) of curves with finite energy were obtained. The chordal Loewner energy a priori depends on the orientation of η, namely viewed as a curve from 0 to ∞ or from ∞ to 0. However, the second author [23] proved the direction-independence (or reversibility).In this paper, we generalize the definition of Loewner energy to simple loops on the Riemann sphere γ : R →Ĉ where γ is continuous, 1-periodic and injective on [0, 1) : We just observe that the limit when ε → 0 of the chordal energy of γ [ε, 1] in the simply connected domainĈ \ γ[0, ε] exists in [0, ∞] (Proposition 2.8), and define it as the loop Loewner energy of γ rooted at γ(0). Note that circles have loop energy 0. Intuitively, the loop energy measures how much the Jordan curve γ[0, 1] differs from a circle seen from the root γ(0), in a Möbius invariant fashion. The loop Loewner energy generalizes * the chordal Loewner energy: Indeed, if we apply z → z 2 to a chord η from 0 to ∞ in H, the positive real line together with the image of η forms a loop γ through ∞. It is clear that its loop energy rooted at ∞ (i.e. we parametrize the loop such that γ(0) = ∞) equals the chordal energy of η.Note also that the loop energy neither depends on any increasing reparametrization of γ fixing the root, nor on the direction of parametrization. The latter fact basically comes from the chordal reversibility, which can be used to show thatγ(t) = γ(1 − t) has the same energy as γ (see Section 2.2 for details). But it depends a priori on the root γ(0) where the limit is taken, not only on the Jordan curve γ[0, 1]. However, our first main result states:Theorem 1.1. The loop Loewner energy is root-invariant.In particular, this result shows that the loop Loewner energy is a conformal invariant on the set of unrooted loops on the Riemann sphere, which attains its minimum 0 only on circles.In our proof of the root-invariance, we approximate the curve by well-chosen regular curves and are led to the following question: What can we say about the relation between the regularity of the driving function and the regularity of the curve? Prior to this work, only one direction was well understood. Slightly imprecisely, the following results state that C α driving functions generate C α+1/2 curves for α > 1/2, where C α is understood with the usual convention as C n,β , where n is the integer part of α and β =...