Seaweed farming has the potential to produce feedstocks for many applications, including food, feeds, fertilizers, bio-stimulants, and biofuels. Seaweeds have advantages over land-based biomass in that they require no freshwater inputs, little or no fertilizer, and no allocation of arable land. But seaweed farming has not been practiced at scales relevant to meaningful biofuel production. We describe a techno-economic model of large-scale seaweed farms and its application to the cultivation of the temperate seaweed Saccharina latissima and of the tropical seaweed Eucheumatopsis isiformis . At farm scales of 1,000 ha or more, our model suggests that farm gate production costs in waters up to 200 km from shore are likely to range between $200 and $300/dry tonne . The model also suggests that production costs below $100/dry tonne may be achievable in some settings, which would make these seaweeds economically competitive with land-based biofuel feedstocks. We also outline a techno-economic pathway from small-scale (1-10 ha), semi-mechanized farms that are viable today to large, biofuel-scale operations.