Strained silicon is now an integral feature of the latest generation of transistors and electronic devices1, 2, 3 because of the associated enhancement in carrier mobility4, 5. Strain is also expected to have an important role in future devices based on nanowires6 and in optoelectronic components7. Different strategies have been used to engineer strain in devices, leading to complex strain distributions in two and three dimensions8, 9. Developing methods of strain measurement at the nanoscale has therefore been an important objective in recent years but has proved elusive in practice1, 10: none of the existing techniques combines the necessary spatial resolution, precision and field of view. For example, Raman spectroscopy or X-ray diffraction techniques can map strain at the micrometre scale, whereas transmission electron microscopy allows strain measurement at the nanometre scale but only over small sample areas. Here we present a technique capable of bridging this gap and measuring strain to high precision, with nanometre spatial resolution and for micrometre fields of view11. Our method combines the advantages of moiré techniques12 with the flexibility of off-axis electron holography13 and is also applicable to relatively thick samples, thus reducing the influence of thin-film relaxation effects.